Fit is important on a tri or TT bike. I’m 6’2" and used to be about 205 - down to 185 now (thanks TR). I had an old Cervello tri bike from 2006 that I got back into racing on last season. I got that bike in a 58 inch size around 2008(because hey I normally ride a 58) - thats what I had so it’s what I raced on last year. I was interested in a new bike and got a used frame that was a few years old I’m building up over the winter for next year.
Tri bike fitting is all based on stack and reach. Picture a graph with an x and y axis and the origin is your bottom bracket. reach is the x value and stack is the y value to reach a measurement on the frame that correlates to your positioning of your forearm pads or headset(different manufacturers I think can measure this a little different based on their proprietary definitions) but most often its to a point on the pads. The idea is once you know your stack and reach you can figure out what bikes and sizes work for you.
Before I got my new frame I got a bike fit on my old bike to figure out my previous stack and reach. Also had a “guru” bike fit where the fitter put me on a computer adjustable bike and he adjusted different positions for me and then they have a database of bikes and sizes they recommended for me. It was actually very enlightening as despite being tall and lanky with long arms I have a relatively short reach and fit better on smaller frames. I ended up on a medium 54" frame and normally ride a L/XL 58/59 inch frame on road and mtb.
I just finished up most of my build and got my new ride on the trainer and it’s spot on.
A couple hundred bucks on a bike fit to figure out your stack and reach is a good investment before dropping big coin on a new bike.
check out slowtwitch.com forums and you will go down a never ending rabbit hole on tri bike fit discussions