If you have a windlass, an 8-plait rode is what you want (rather than a 3-strand twist).
If you are anchoring without a windlass, either 3-strand or 8-plait will work.
For a 21, you would be fine with 3/8" or 7/16". 1/2" is very good but probably overkill unless you anchor regularly in snotty conditions.
Using 3/8 or 7/16 ths, you can put more than 300' in your rode locker.
Length depends again on the conditions you anchor in.
1:7 in snotty conditions. 1:3 or 4 in mild conditions.
At 1:7, 300' of rode is only good for about 43' of water.
At 1:3, 300' will hang you in 100'.
So what sort of conditions do you generally anchor in?