I use 4.5- from my experience I believe most people do. I have bought some big geckos that have needed the 6 and/or 9 inch kind but I've only sent out geckos that were up to 60 grams. I use SE also.
SE is def. they premiere place to buy d.cups. I would go with the 4.5 unless you have Super G's, or Really large geckos. The larger ones have more room and I have had nothing but trouble with shippments coming in with loose, banged up, and/or injured leos because someone decided to use a larger deli cup.