Finding dock space

It depends on where you are. Some areas have kept up with the ever increasing boat size... others not so much

Not necessarily, maybe a tad harder at times. Depending where you want to go with the boat. But most places have dockage for a boat that size in any given area. I'm an Mid- Atlantic boater by the way.
If you are doing the great loop or some travel up/down the inland rivers it can get perhaps a little more challenging. I'm not totally sure on that, other that really know will chime in.

I think 58 -60 you are pressing your luck just a bit. Calling ahead reservations long before will be your better resource.
Were 58 and the very few times we need to tie up on short notice for the night, it's usually the fuel dock and a promise were off early.
Canaveral Marine has our CC on file just for this.

I agree with Pascal. It seems marinas in FL have all been run over by hurricanes or investment companies and have rebuilt to new larger slips with wider beams. Mid atlantic is a mixture of new and old. Great loop might be harder in some areas, but a lot of transient dockage is all side tie, so total length matters less.

I think the beam matters more. Seems like slips wider than 16-18' are harder to come by in some of the older marinas.

Basically if you're running under 50' you can generally just roll in and find a slip. Once you cross 50' it's a real good idea to call ahead and reserve.

I'm going to note that your question was strictly about transient dockage. It might be slightly more of an issue simply because a 34' will fit anywhere, but there is really no issue finding transient dockage for a 58' boat. In some areas, it might limit you to one of the two or three marinas or to two of the four marinas. We did the loop in a boat measuring 69'4" and had no real issue with dockage. Worst case it steered us a few miles away. Now, we docked at many marinas that could not have accommodated us on a full time dockage basis as they didn't have slips large enough.