It varies depending on whose kitchen you're using. If you need more space, you just have to put less things in your rom. I know calkulin's kitchen's usually ended up with less space than groove's with the same OEM's cooked in (ones with option.xml's for the old style). Calkulin included some things in the 'System - name' OEMs (were not selectable in buildos) that groove did not use. If you are using visual kitchen, you can use lzx compression easily to get more space with the same amount of stuff cooked in. I don't know the drawbacks to it, as I haven't used it. If there's rings/pictures/whatever you don't use, delete them from your kitchen and you'll get some space.
You can also not cook in an OEM that has a cab install. If the .exe is large (like GarminXT in my case), I cab install it and use
squeezer to comrpess the exe and save some space. This is one area I sometimes wish I had a touch pro - double the space
.
Also, make sure opera cache is on internal storage and store email attachments there as well.