Our shower is 36x60 and does not feel overly long, and if it was a foot longer it would still feel fine, especially if the bench is at the end of the shower.

Are you changing the shower anyway?

Do you have the room to keep the width and just extend the length? To me the 36 inch width is very small unless this is a guest bath not used often. I wouldn't put in a shower less than 42 inches wide in a master bath and 4 ft wide is better. JMPO.

I haven't had the experience of a drafty shower. I have had very large showers in previous homes (largest was about 8'x8'). However, I'm in south FL so having a drafty shower isn't really a possibility here. I have shown new construction homes with that very long shower that gets negative responses from the buyers because it went from a reasonable length like 6 or 7 feet to something like 11 or 12 ft. Totally ridiculous. The shower has to fit the room. You do have a designer so that is a good thing. The thing that I disagree about was the very skinny width your designer suggested. Is that because of the size of the room or the location of the shower or something else?

I agree with Denita to keep the wider width. One way to minimize the possible draftiness would be to continue the glass shower enclosure up to the ceiling, in which case I'd properly tile the walls all the way up and be sure the tiled walls and ceiling have moisture resistant drywall so you wouldn't create problem with deteriorating drywall. The length of the shower should not affect draftiness. I'd go as large in either direction as you can comfortably within the available space.