I hit up my cousin, she's a wedding planner so figured she might have an idea. This is the formula she uses so hopefully it helps
Up to 100 people - 150sq ft
100-150 people - 400sq ft
Cocktail - 8sq ft per person
Seated - 10-12sq ft per person
Hospitality Unit, Bar & Dining 18-20 sq.ft per person
That sounds ridiculously big to me and she said she's only done 1 marquee before so not sure how accurate it would be.
If you're hiring a marquee the company should have a rough idea.