There are two possible meanings for "bigger" in this context, so I'll mention both.
1) Physically bigger settlements, i.e. building outside of the green areas. There are mods for this, but none are guaranteed and problems may result.
Place Everywhere allows you to place anything anywhere, so you can build something inside the green area and then move it outside the green area and place it there.
http://www.nexusmods.com/fallout4/mods/9424/?
Build High initially just greatly reduced height restrictions (buildings can go up to ~50 stories high), but it now also expands the boundaries of 16 settlements as well.
http://www.nexusmods.com/fallout4/mods/3528/?
I vaguely recall seeing a mod that allows you to build a workshop anywhere, which would make it possible to build a settlement anywhere (a settlement is tied to the workshop), but I can't find it.
2) A bigger limit to the number of items that can be built in a settlement. This doesn't even need a mod - you can do it with a couple of console commands in each settlement. I used the "Higher Settlement Budget" batch file to make it more convenient:
http://www.nexusmods.com/fallout4/mods/818/?
Somewhat related to the item limit and definitely useful in building is the ability to scrap almost everything in a settlement. Trees, paving stones, ruined buildings, leaves, skeletons, weeds...almost everything.
I used Spring Cleaning for that because it's in active development and avoids the cell reset bug that is normally triggered by scrapping things that can't be scrapped in the unmodded game:
http://www.nexusmods.com/fallout4/mods/4640/?
You will probably want more buildable objects to choose from when building. There are a vast number of mods for that, but there's a problem. For no known reason, Bethesda limited the number of keywords in the menu to a little over 200. If you install mods which add items to the building menus, you can very easily exceed the very low limit, which will result in loads of buildable items being unavailable to build. They just won't appear in the menus. You can easily lose entire categories of items (for example, I lost all lights apart from candles and campfires and all the containers you can build in the unmodded game). It's a huge pain in the backside.
There are modding resource projects which aim to work around this rather strange limit by creating a limited number of new keywords in the hope that other mod authors will use only those keywords. When this works, it results in a vast amount of scrolling back and forth in menus because the limited keywords result in dozens or even hundreds of items in the same submenu, but at least you can build them all. In my experience, it doesn't work at all because not all mod authors restrict themselves to those keywords. Since the keywords mods add many keywords themselves, I found that it made things much worse.
I'm belabouring this point because it's a major restriction for no obvious reason and there is no indication of it at all in the game. Stuff just disappears from menus, so you'd probably assume incompatible mods.
Personally, I use Homemaker because it has loads of stuff for building and it has a practical menu structure.
I also use:
1x1 craftable foundation pieces. Often even more useful than the 2x2 pieces available in the unmodded game, although they only snap to each other on the sides and bottom so you have to start building from the top and that's not ideal.
Better bed mattress textures. Who the hell would use the filthy mattresses in the unmodded game? People would sleep on clean straw or even grass rather than that. We're settlers, not raider scum.
Craftable elevators. Which I've never used. But I might.
Craftable ramps and rails. Likewise.
Craftable interior shack doorway wall. Useful if you want to make rooms inside a building, although Homemaker now provides the same functionality and a doorway without big holes either side. We may be scavenging, but basic carpentry is a skill that can be used with simple tools and wood should be plentiful in this area of the Fallout world.
JC shirt and jeans for vanilla body. I just wanted a basic mod to make clean craftable clothes for settlers and this was the least soft porn one I could find. Unfortunately, it makes "HEY I LOST HALF MY SHIRT BUTTONS LOOK AT MY ****" shirts for women, but I equip my settlers with combat armour and the chest piece gives more practical coverage. It makes a clean jeans and T-shirt for men because it's a mod of that outfit that is made only for women (and so provides the default for men). The state of clothing mods for Fallout is pretty sad unless you're an adolescent sexually attracted to women.
Lore Friendly Posters. Adds a lot of buildable wall decorations in the form of posters made from the covers of the magazines in the game.
Stackable concrete foundations. Adds snapping points to concrete foundations so you can stack them neatly and easily. Very useful, especially for building walls for fortified settlements.
Simple Intersection. Slightly decreases the clipping point, at least for some items, making it slightly easier to build some things without gaps.
I also use a few minor gameplay mods:
Disable Minuteman Radiant Quests. Both the settlement defence version and the Preston and Radio Freedom version (Shut up, Preston!)
Texture Optimisation Project. Improves performance and fixes stuttering with no apparent reduction in graphics quality apart from a high zoom on very close objects. You probably won't need it with your hardware, but my kit is somewhat older.
Full Dialogue Interface. For people who don't mind reading a few words and would like to choose what their character says rather than choosing a word that might or might not be related to what their character will say.
Shaikujin's Better Alerts. Gives you a more visible alert if a settlement is attacked, one that you have to acknowledge rather than one that briefly appears on the edge of your screen that you might well not even see.
