note also that the bar needs to long/wide enough for (the mount that you plan to use) the weight of the whole setup.
just as well, if you plan on using an in frame engine you need to know that typically a bike with rear suspension will not work for this as you typically require the use of a v-frame bike. the bigger the better is always what i remember but, keep weight in mind as 5 lbs. can make a huge difference. so, you need the bike to be big without being heavy.