No worries. From the look of this bunch it would be possible to make a nice slab forest… or a couple clumps, or some singletons
This entirely depends on the choices made. In any case it would be good to chop and zip tie or wire up a new apex. Where the cut is made depends entirely on the goal for the tree.
Dunno height exactly as the tree are always cut back to keep a desired shape. There is one at the Pacific Bonsai Museum that is est. North of 4’ tall.
However the better question is what is the diameter above the nebari. Lots of folks like at least 3” diameter before they will even start working a tree into a training pot.
Again, it depends on the goal(s) for the tree(s). But if thicken is desired one must up pot yearly and work the roots each year. Usually the bottom roots eventually fuse. … And this section need to be sawed off.
Up potting with little root work would be ok this year. However it is too late to repot. Do this next year as the new buds turn slightly green.
Hope this helps.
Cheers
DSD sends