Tri Bike Fit

Hi folks,
I ride a Specialized Tarmac Comp as my road bike, size XL which is 61cm frame and I love it. I'm 6ft4.
I've just bought (via an online retailer so limited opps to test ride) as Specialized Transition Expert. This is also an XL frame but this time only a 58cm frame. In my mind, this makes thebike feel realyl small and I wondered if any experienced gear heads out there could comfort me over this concern? Would I expect all TT frames to be a touch smaller than road frames? Does this 3cm difference in size matter?
I have yet to have a proper bike fit which I'm sure will help - none of the spacers are currently fitted etc and the seat options seem numerous.
Given the cost of thebike I do feel a wee bit sick about it, so honest words of wisdom appreciated!