If you've got little or no experience then a 6.5 may be tooo fast to begin with!
I started with a 10.5 brushless in a B4 and found it to be great place to start. When I put lipo in it for the first time I even backed off the top end on my transmitter to limit it and gradually turned it back on as I improved.
Much crashing, breaking stuff and loosing races to begin with but slowly improved. A few years on and I'm now running a 7.5 in an X6 (converted B4) and it works great for me!
I ran with a 10.5 once at Tiverton, and was easily as quick as the pack in the tight, flat infield section, and only lost out on top end on the main straight and steep gradient sections.
6.5 and 7.5 seem to be a popular choice for the top and more experienced guys, but plenty of people run lesser motors 8.5/10.5...for newbie's slower is definately quicker!!!
Hope it helps and enjoy, Tiverton is a great fun track, a lot of 2wd's, inparticular B4's (it was when I was there at least!) and good mix of abilities! You shold enjoy it
