Hevi-beads tend to chip and sometimes crack if fished directly on the bottom, I'll use Hevi-beads if I fish a small piece of bait such as roe or dew-worm.
If I am fishing bead only (mostly when fly fishing) I'll go with 6-8mm trout bead for clear conditions when targetting steel and larger for when the water is colored or faster flowing, but if I do that I'll run an indicator or a slow sinking like 1.5-2 IPS with a fluro leader and fluro tippet.
If you are fishing with a float using a 2-3 foot leader is my recommendation, I know some people who use longer but it depends on how you are fishing and where. If you are hanging up/ snagging bottom it could be because your hook is large or fairly heavy causing it to basically act as an anchor. Either that or your bead is fairly small like a 6mm and might be getting wedged in areas where rocks are roughly fist sized. If I run a bead to run on the bottom of the river I make sure the area I'm fishing is usually sandy or gravel bottom to prevent hang ups.
As for size... this is sort of a match the spawning fish/ timing thing or at least for myself, springs have large eggs so anything over 10mm is ok, chum 8mm, coho and steelhead 6-4mm.