Is a bushel a weight measurement or a size measurement?

Answer

A bit of both. It used to be a volume measurment, but today its used as a weight measurment of 56 lb.
The above answer is absolutely wrong. I own thousands of acres of fruit in the Indian River Fruit area and the domain name indianriverfruit.com so i should know. a 1/4 bushel of fruit as are all peck and bushel measurements is a volume thing. a 1/4 bushel of fruit will weigh 10 to 11 pounds a 1/2 bushel 20- 22 pounds a 3/4 bushel is 30 to 33 pounds and a bushel is approx 40 to 44 pounds.
take a bushel basket of oranges and weigh them and you will find I am right!
Hope this helps!

Depends on what you are measuring or weighing---ie bushel of shelled corn is 56 lbs.
bushel of southern peas is 25 lbs etc.---my 2cents.