How much can I trim my crypts roots?

Yes.
When I receive new plants (or redo the old) I regularly trim back the roots to a uniform length of as short as one inch. It makes replanting ever so much easier, and can actually rejuvenate the plant itself. You may see some slower growth of the foliage for a bit while they regrow their roots, but that will be very temporary.

Again agree w/Toni. Trying to manage and plant long root strands is IMHO counter-productive and endlessly frustrating. On swords and Val I do also reduce the foliage a bit to a good bit at the same time - but after a brief pause, recovery is clean and healthy. For most individual crypts I don't remove much foliage unless my clumsiness has broken some (not rare).

I have noticed that simply moving Crypts within a tank will cause *SOME* melting, but it will be limited. Trim the roots to an inch or so, and they will be fine.

Moving Crypts to a totally different tank with different water and soil conditions will result in a more massive melt.

I agree with most of the above, however Crypts. require a bit more caution when pruning roots, than say Swords.
When you pull the plants you will notice that Crypts. have a tendency to grow long white, almost bare roots and also some shorter roots with fine root hairs on them.
I cut the long, bare ones mercilessly, back to an inch or so. I've found that many of the shorter ones with the 'hairs' on them are a lot shorter to begin with, and I try to be a bit less radical when pruning them back.
As with trees, I feel that the long ones act more as anchors, and hairy ones more as feeders. I think it is very important to try and cut them as uniformly as possible.
IME, Crypts. are not as prone to melting when re-planting within a tank, as newly purchased ones from the LFS, or to another tank as Torf describes above. In fact, I have yet to have one melt when re-planted in the same tank. Just my experience........

Len

I do agree w/Len on Crypt melt, but I don't even get melt moving between tanks. However, all my water is much the same, and almost all the lighting is quite similar. But new and especially emerse-grown crypts are much more likely to melt.