Trailer length - does that influence spot availability?

If you have a dozen or so specific campgrounds you expect to visit most often, visit their websites to see what if any limitations they post there.
While you can usually find a place with most units, if you are going to be in older campgrounds, especially older Government owned campgrounds, longer than 30' can become an issue. At commercial, usually not, as they want every spot filled and will arrange spaces to ensure maximum usage.