As suggested by Steve, you can have a centered square datum curved of 30"x30" in your start part template. When you create new part with default template, that square will appear there. This will lock your sketch envelop to datum curve size i.e 30"x30". Later on after creating basic construction features (without referring first datum curve),you can delete datum curve feature.
Hope you are getting what I mean to say..
Cheers..
Yogesh Birari