Hi Dallas
Great question and one a builder should consider every when they build a room.
Katsu is right in that the 64bit version of the game will help with lag but a lot of us have been using 64bit for a while now via Alex Modz so its kind of already tested.
My advice when building a room....first you have to ask yourself what are you building!!
If its a house for you and a few friends then 1500kb is fine, if its a nightclub and you want 100+ people there...keep it below 700kb
Also a note on the file size.
Every object you add to your room adds to the file Size (the actual physical Size of the build has little to do with it)
So, Small details dramatic increases File Size so you have to ask yourself...Do I really need to make the most awesome DJ console for my night club with 150 buttons and flashing lights....that is 230KB!!! or do it just use the default DJ console because...only the DJ will see it anyways.
I have seen night clubs where a builder has spent....days creating the most amazing feature on the roof...but in 3DX...hardly anyone looks up!!
I have seen night clubs where the builder has hand made 300 different bottles for the Bar....no one is sitting at the bar.
I have seen houses where the builders has made the full internal workings of an oven then....closed the door so you can not see it.
Lag does restrict us and it varies greatly depending for every user and what their computer can handle.
As a builder, you need to learn to manage your build and put in the details where it will have the most effect.
Nym