How do I calculate how big my attic conversion could be ?

I see various threads stating how much per square foot their conversion is costing and I'm at odds to work out how many square feet my own could be. Bearing in mind I have a trussed roof also, does that effect the calculations ?
I'm hoping to get some quotes in over the next while so just trying to get my head around it and not seem clueless when the builders come a calling.
TIA