How is the size of the 240 volt double breaker (the one that protectes the sub panel)at the main panel determined? Exp. Clothes dryer at 240 volts/30 amps-two 20 amp ckt.- one 15 amp ckt. Is it the possible total current draw of 85 amps, if so than the feeder wire will be what size?

The main panel is located apox. 25 ft. from the sub panel.

The size of the breaker is determined by the size of the wire.

The size of the wire is determined by the required feeder size.

The feeder size is determined by a load calculation, OR the knowledge of an experienced electrician.

You do NOT add up the breakers to see what the load will be.

The size of the breaker is determined by the size of the wire.

The size of the wire is determined by the required feeder size.

The feeder size is determined by a load calculation, OR the knowledge of an experienced electrician.

You do NOT add up the breakers to see what the load will be.

How is the load calculation determined?

Your sub panel should have a rating like 100 amp you need to match the rating of the sub panel feed breaker with the rating of the box.

your sub panel should have a rating like 100 amp you need to match the rating of the sub panel feed breaker with the rating of the box.
Not true at all.
A 100A panel can be fed with anywhere from a 30A feeder to a 100A feeder.