I would say, if you have an office chair or something other that is adjustable to sit on( a jack perhaps), take some simple measurements of what you would like from that, and sketch something you would like based on what ever you have seen as an example.
For my build, this is kinda what I did. I had looked at a number of bikes before I had made any thing. Then I had sat in my office chair, with my feet propped up in a good position that I liked and was comfortable. took a measurement of the height of the seat and length of feet stretch.
A start of the drawing.
Then I had held my hands where I would like to have my bars at, a piece of bar, broom handle or whatever helps visualize, take a measurement of that height and relative position of that location on the drawing.
Now you have a good start on what the bike is going to look like. fill in the rest of the frame of elements in your examples you liked.
Now roughly sketch in the tires and other elements you liked, keeping in mind of what kind of engine you are planning to use (ex. a briggs or a clone). For example, many people like the look of a separate gas tank (a briggs typically) and others dont like it (as in having a clone engine), this also may influence the size and shape of the frame and also other aspects of the bike.
I believe this may help you start in gathering some info. If you aren't able to get any dimensions from people, I can give you some measurements if you want, but pm me first.
hope this helps.