Im kind of re doing my garden, and dug up some perennial geraniums. Their roots are huge and spreading everywhere! Any suggestions for a good place to plant them so they don't crowd everyone else? Zone 5a NH

You could put then in a place where they have lots of root competition, less water and/or poorer soil, rather than an open bed with root free rich soil. How about in the woods or under a tree.

You can also chop the roots off to a manageable size. The geraniums won't notice.

Before you replant them (unless it's too late ) do these geraniums seed around? If they do, I would recommend not replanting them or ultimately you will have huge rooted geraniums all over your garden and not just this one to relocate
There are lots of perennial geraniums that don't seed around - 'Orion', wllassovianum, and 'Hollywood', come to mind. One species NOT to buy because it seeds around is geranium phaem and all its cultivars.
Of course you may like rampant seeders