" Are they too small for cherries? I read that tomatoes needed huge pots or land for their roots."

That all depends on the specific variety of tomato you plant. The size of the fruit has nothing to do with the size of the plant.

Many cherry varieties are the biggest tomato plants in existence - 8-10 feet tall and 4-5 feet wide. So yes, your pots would be far too small for them. But there are some varieties that are quite small plants and would do fine in those containers. So what is the name of the variety you planted?

In your location most would buy already growing transplants in order to have time to get fruit before frost as you have a relatively short growing season. Direct seeding into a container as you have done is a very unusual approach and full of problems. Honestly, it just doesn't work for a number of reasons that would take pages to explain. Can you buy some plants instead?

Using compost in containers, using fertilizers, and using sawdust are whole other discussions so let's see if you actually get some plants first and then worry about fixing those concerns, ok?

Dave