i  have  10'  flat  bottom  jon  boat   and  want  to use  a  trolling  motor  as  my  main  motor what size would  i  need  to effectively  push  the  boat?

You can move the boat with a 30lb thrust Minn Kota Endura but if you wanted something faster just go up in thrust. I used the 30lb on my old 12' aluminum boat and it worked great but it wasn't my main motor for getting around. 

Thank you  for the  input i was thinking at  least a 30  but  being  it  is going  to be  my main  motor i  will probbaly go 55 

definitely 55 lb 

Yeah I wouldn't go less than 55lb if it is going to be your main propulsion.  If you get caught out on the lake in a stiff wind you are gonna wish you had more power most likely.

I run a 10 foot Tracker jon boat with a 30 lb Endura on small electric only lakes up to 100 acres or so.  It works for the most part though a strong wind can slow me down a bit.  I think the Endura Max 40 or 45 lb options would suffice.

If this will help, I have a MG 55lb 12V, digital variable speed on my 17'6" aluminum and I can stand on that thing all day on the Potomac in wind, current and tide and never run out of juice.  I store in a state park and so the 12V battery has to come home with me each outing to get charged up and that is why I do not have a 24V TM.  I use the engine to get me to a spot and then hours on the TM moving around up the big tributaries.  It is a big river.