I say try Vistaprint. I'm using them and I got great quality at an unbelievable price. I orded 50 and only paid about 28 bucks for them. You have to catch them at different times because sometimes the prices are different depending on what specials they are running.
I have not been to Rexcraft but if you look at the left hand side of their website, they have 265 reviews and they are at 4 1/2 stars. That's not bad. A 4x5 invitation is not too small. Invitations are the size you choose, don't worry about size as long as you are happy with them.