I'm building a world too (well, it's just Riverview's map with some changes) and I'm using a lot of custom sized lots because I like how they suit Riverview's downtown. I usually think custom sized lots are better for aesthetical purposes, but they are extremely impractical when it comes to placing pre-built stuff in them or uploading the lots to the swap shop. I now have a 63x11 lot in Riverview (a park by the river) and it looks really pretty, but there's no way I could upload it since no one would be able to place it anywhere unless he would load the world in CAW and place the lot there.
I think I prefer custom sized lots when I am creating the world and standard sized ones when I am playing it, since that means that if I don't like something 100% I can easily change whatever I want without spending hours trying to find a building that suits.