I want to replace 3 support posts with a beam that will span 24 feet and be supported at each end by a suitable column. this is in a garage that is 24 x 36 feet and made entirely from oak lumber. the beams would support a typical 4/12 roof system with rafters and ceiling joists made from oak lumber 16" OC. the posts that I want to replace are approx 8x8 oak and they support an oak beam that runs the length of the garage (36 ft) the beam is made up from two full dimension 2x10 oak planks sandwiched between full dim 2x4 oak. I would like to span the 24 ft under the 2x10 bem in the same location as the current support posts. Can anyone help me to determine the proper size beam to span this.

Yes indeed...a good architect or a structural engineer would be the person from which to seek help. You have not provided enough information to determine a beam size, and I, for one, believe it is improper and unethical to provide structural design services on this site, especially without actually seeing the conditions, and with insufficient information.

I will tell you that from what you have said, and making some assumptions (which may or may not be correct), you are looking at a steel beam or a very very large glue-laminated wood beam. Then the question is, what will support this beam at its ends? A "suitable column" is fine, but what supports the column? The loads on them are likely to be great, and a serious footing under each column will likely be requred.

You say the oak beam is 36 feet long, and then you say you would like to span 24 feet. If this is correct, you are talking about a 24-foot beam with a large concentrated load at its center in three locations. Do you have enough headroom for a beam that could be approximately 16 inches deep? Are you prepared to dig down and place six concrete footings under the ends of the beams?

There's a lot involved here. Professional help is required.