Trailer length - does that influence spot availability?

According to someone who compiled every federal campground spot a few years ago at 35' you still officially fit into 85% of sites, every foot over drops you around 4-5%. State spots are generally a bit more lenient but it's not that far off.

Heck, if you make reservations last minute like I did for my current trip every foot counts. We got the last spot at 3 campgrounds, 1 would fit up to 24', the one we're on right now is super tight at 18', the one tomorrow would fit 30-31' but no longer than that.