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

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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