progression mission question

[the taffer]

I was thinking of making a progression type FM, but I want to make a big world instead of just making the world as I go along. I do not see any problems with this but I want to make sure that I can't run into anything and have to restart. My reasoning for doing this is because im adding lookout towers near a city and I want those towers to see everything. Also I think it would be cool if you can see a lot more when your on a second floor looking out a window. This way will be harder if I do decide to do it but if it makes a mission look better, I will do it. Any suggestions will help.