Without seeing the actual design, it's a little difficult to say, but the basic premis of an A-frame is that the two main members of the structure are in either tensision or compression. 2" square tubing of either stainless steel or aluminm would be plenty strong (I'd probably shoot for aluminum since it's lighter). Painted carbon steel would be pretty cheap, but would be subject to rust. If it didn't have to telescope, I'd say look into using pressure treated lumber. As for wall thickness, you're going to be limited to what you can actually find- probably about 1/8".

For reference, take a look at you trailer. The cross beams are probably 2" steel square tubing and they're capable of carrying the entire weight of the boat (or boats + people, etc.) in bending. Additionally, the main frame of the trailer is probably only channel.

sm