I an wanting to run power to my ultitly bldg, can I put a small breaker box in the bldg my breaker box is about 75 ft from the bldg and has 7 open spots for breakers. Also what size wire would be best . I would like to have a couple 20 amp plugs and overhead lights

You are going about the process backwards.

You need to start at the utility building first. Determine the maximum load needed for what you will in the shed. Lights, receptacles, motors, heat or a/c, whichever is the highest.

Once you determine the load in VA (volt amps) then you can work on the sizing of the cable over the distance you need.

Also, some other things you need top be aware of, you will need a separate grounding system at the utility building. That means one or two ground rods (Based on the resistance of the ground near the shed)

At the utility building the cable from the Main Panelboard must be a 4 wire cable. You will need either a Main Circuit Breaker controlled panelboard or a Main disconnect switch in the utility building. You cannot use the "rule of 6" because you are wanting at least 7 breakers so far.

Get us some answers and we will be glad to help you.