Sprouting wheat in buckets, help?

I don't think it matters very much what size drill bit you use. You don't want the dirt coming out of the holes... are you planning to use shards of broken pottery to cover the holes and gravel for drainage? If so you can do with two or three large-ish holes (1/4). If you are not planning for gravel or shards of pottery to aid drainage, then do four to 6 holes that are slightly smaller.

I hope this is helpful,
Jenny

 

what size a wheat grain is

Rather then worry about making small holes, drill them at least 1/4 to 3/8 inch and lay a piece of screen or cloth on the bottom of the bucket​

 

Coffee filters work well for that, too. I use them in the bottom of my hanging baskets, over the drainage holes of my indoor potted plants, etc. They also make great holders for tacos (especially for children), and great drip-catchers for popsicles. They can also be used to polish crystal & glasses without leaving lint. I use them for mirrors, too.