Here are some before/after screenshots with two shadow settings so you can estimate what settings might be suitable for your machine.

Skyrim shadows before after

iShadowMapResolution 512 vs 4096

As you can clearly see the shadows on the high resolution pictures are a lot sharper and look much better.

screenshot with low shadow resolution skyrim

screenshot with low shadow resolution skyrim

 

iShadowMapResolution 512 vs 4096

512: If you look at the shadows further away, they look “ok”, but the shadows close to you have a lot of rough edges

screenshot with low shadow resolution skyrim

4096:

screenshot with low shadow resolution skyrim