When to use what size weight?

For worms,craws ect.....

i have always just used 3/16oz bullet weights, but what situations call for less or more weight?

many factors: Wind, current, depth, type of cover your fishing...etc

try to get away with the lightest weight possible!

check out these links:

http://www.bassresource.com/fishing/worm-fishing.html

wind is a big factor as well as the worm you're using.  I like to go as light as the conditions allow.  For example I like using the berkley 7" powerworm with 1/8oz so that as it's falling down, it's doing it slowly.  Just a thought

Any time I t-rig plastics, I use the least amount of weight possible that will get the job done.  That can vary dependant on the thickness of vegetation/cover, depth of water and the rate of fall I want.  For a slow drop in sparse cover I'll use as little as 1/16 oz, or for crashing through thick hydrilla I'll use up to 1 and 1/2 oz.

Basically, let the fish tell you what weight to use.  As long as you have enough weight to get through the cover you're fishing, or little enough to rest on top, you can vary the amount of weight you use to increase or decrease the rate of fall of your bait until the fish start biting.

thanks for the information,

for texas rigged worms, i use 1/8 oz green or black tungsten weight that is unpegged about 90% of the time.

When fishing sparse to moderate cover, I mostly use 3/16 and 1/4 oz unpegged weights.  Sometimes I'll use 1/2 oz and heavier pegged weights and fish with more aggressive snaps to go for reaction strikes and cover a little more water.

In heavy weeds, I'll pitch plastics with 1/2 and 3/4 oz pegged weights to punch thru the weeds.  The bulrushes, coontail and milfoil I fish aren't super thick like the weed cover in some parts of the country, and I rarely need to go heavier then 3/4 oz.

ROF 

I try using the lightest possible and still have contact with the worm.  couple of my favorite pond rigs are 4" and 7" Powerworms on 1/64 and 1/32 oz weights.  Throw these on med ex fast spinning gear with 10lb line.  Puts a hurt on the fish. 

I used to start with an 1/8 or less and work up. Recently I start with 1/4 or 5/16 and work up or down from there. If the fish will bite a faster falling/moving worm you can cover a lot more water with a heavier weight. If not it doesn't take long to try a lighter weight.

If cover is sparse I prefer a split shot or weightless rig for an even slower fall, because it seems the slow fall is what triggers strikes. As the others said, the lighter the weight you can get away with, the better, USUALLY anyway. Punching through weeds with heavier weights works wonders though.