Just try and try, it's not so hard as it looks. I was in the same situation about one month ago and now look my signature, it's just a power of will.
Actually my main problem was that same sizing thing and now it's one of the easiest parts when I know how I do it.
For example, just download the attacker's KTM 2013 blend release (easier to fit parts when you see a well made model dimensions) and append your model there all parts in one before sizing them. Make it fit to the bike or how ever you want. I'd make sure the main parts axles are visually aligned, use your own mind on this.
Then you have to make sure that your model and the base model pivot points are at the same points.
For that select one of the base model parts then go to Object -> Snap -> cursor to selected. Then select that same part of your model and press Shift + Ctrl + Alt + C -> origin to 3D cursor.
Each of your separated parts must have same pivot points as the base model parts has. (Upper and lower fork, frame, swingarm). So do these things for each parts separately.
Hope this helps somehow
