Hi all
Looking to buy road style commuter bike, but what size.
I'm 6'4" with inside leg 35"
My current mtb frame is a 19" because I prefer the greater stand over etc.

I've noticed that more modern road frames have adopted the sloping top tube mtb style of frame, but I do like the look of the Charge "retro" look

conventional 62-64cm I'd say, or look for top tube of the same length.

Probably about the same build as you – I've got a 60cm Kona that fits me fine. Think some of the bikes don't come much bigger than 60cm, depends what budget you've got.

Standover isn't as much of an issue with road bikes, you're less likely to have step off it quickly, chuck it around etc.

How do you get a seat post long enough to ride a 19" frame?

I'm about the same height and have had 60cm and 58cm road frames. Effective TT is the most important consideration.

Bikefit £30 at your LBS