I have a medium and large Tikit. I'm at the low end of the size range for the large, but prefer it a lot more than the medium. I think the longer wheelbase and being further behind the front wheel make a considerable difference. I can ride the medium, but I feel cramped and frankly it feels like folding bike [a bit small!]. OTOH on my large Tikit I feel like I'm riding a full size bike and really enjoy the more stretched out position.
I had a medium and a large too. I'm 5'10" and fit either. I kept the medium because it fits me better with drop bars than the large, and ultimately I prefer drop bars to flat bars. With flat bars and a 10cm stem the large fit me really well.
Normally I ride 57 to 59cm road frames with a 56 to 58cm top tube and a 9 to 11cm stem.
alex