Any G1/4 sized fittings will work.
As for size of fittings is your choice really, I use 16/10 mm in mine.
So all my compression fittings are 16/10mm, the tube I use is "EK-TUBE ZMT MATTE BLACK 16/10 MM" as it doesn't yellow like clear ones since it's opaque and black.
As for a radiator, depends on your components and your case, if you have enough space go with a 360mm radiator like "
Corsair Hydro X Series XR5 360mm Slim", if you need a smaller one go for something from that series.
Rule is go with the largest one, because a key reason to go into watercooling is the lower sound levels and a bigger radiator helps achieve that (thickness helps to a degree but its mainly the length).
So if you can't go for a 2x360mm, go for 2x280mm, then 2x240mm, then 1x360mm and so on, don't bother with 120mm unless you're doing small form factor builds.
As for a reservoir (assuming you have a pump), any cheap tube reservoir will do like
this. As long as its feeding water into the pump, you're good.
If you want a combo, Corsair also do an okay one "
Corsair Hydro X Series, XD5 RGB Pump/Reservoir", as long as it has a D5 pump should be fine.
