For the receiving blankets (still the correct term, by the way
), I say the bigger the better! Last year when my baby was born I was disappointed to find that the store-bought blankets are much too small for swaddling a growing baby. 30x30 or smaller are great for other things (burp cloth, toss over car seat, etc.), but WOF or larger is much better if you want to actually wrap the baby up!
As others have mentioned, most quilts won't be going in the crib anyway, so go with any of the above-mentioned sizes that suit you.
If anyone wants alternative ideas for a handmade baby gift, I suggest sleep sacks! The nice ones are expensive so I "upcycled" an old fleece jacket to make one, and even though it works great, it looks really frumpy. I would have been thrilled if someone with more sewing know-how made a pretty one for my baby!