It depends on how long you want the sheets to last - there is a trade off here between thickness, lifetime and resistance to dirt.
It's nice to have clean plastic, and one way is to use the thinnest stuff that you throw away when it is dirty - but it gets wrinkly, and that means it tends to get dirty quicker (trails about more, dirt doesn't roll off, isn't washed so efficiently by the rain, difficult to wash by hand - I don't know, it just gets dirtier).
There's no point having thick plastic (500 gauge +) that isn't UV stabilised, it doesn't last long enough to get dirty but it can start to flake in one place before it tears, and the flakes get everywhere. Clear tarps seem like a good idea but the cheaper grades are designed to go in a skip after one job and if they don't say UV stabilised they can turn into a net in one season
At the other extreme is the double stabilised stuff from Spain - brilliant for polytunnels but I'm not sure you can get it thinner than 800 gauge.
Cheers.