I've installed SSDs in lots of laptops over the last 18 months and watched their usage, and I would say that 64GB is the absolute minimum most people should consider.
30GB is the bare minimum I'd consider.
Remember most laptops have memory card slots, you could get a 16gb memory card and redirect photos/videos/documents to that folder
Disable hibernation! Gave me 14GB back! Also minimise the size of the page file, or move it to another drive, as that can take up a lot of room.