I'm a pretty big Firefly fan too.
Well,
Serenity is listed as being 82m long (269 ft.) by 52m wide (170ft) by 24m (78ft) high.
A
Tel'tak (unarmed cargo ship) only has the cockpit and one cargo room. I couldn't get any exact numbers off of the internet as far as the dimensions, but based on the few side by side shots of it and the characters, I'd say its only about 15-20 ft tall (perspective might be off on those shots, hard to say) and about 30 ft long. Those are pretty rough estimates though.
An
Al'kesh (the bomber) has a similar frame, but does seem to be a bit beefier. I'm still fairly sure its smaller though.
So, I'd say the cargo ships are about garage sized, and Serenity would need a football field to touch down in.