I'm not sure if you are talking about openwater or in a pool but I have done 3 open water 5k's. I never wore one for that just because the lake was really gross and those suits are expensive to be using in gross water. They also wear out fast, as far as their ability to reduce drag.

I swam competitively since I was 8 till graduating college last year and I'm really not a fan of them in general. Mainly because they are expensive but with any activity you do, the majority of your success comes from your mental ability. So spending money on a fast suit doesn't seem that logical to me unless you are going to Olympic trials. On the other hand, some people really love them. Those tend to be the ones who bye into it completely because just putting one on they feel fast.

So long story short, enless you have some extra money to spend I really don't think its necessary.

If you do want one though I would recommend the Blueseventy open water suits because they make you more buoyant. I'm not current on the rules for triathlon swim races so make sure you get one that is legal. They outlawed the use of the original competitive blueseventy long leg suits for competitive swimming so I don't know what the deal is for that.

As far as fit. Fast suits very from brand to brand so definitely try them on! Blueseventies, I know, are supposed to take about 8 minutes or less to put on so if it takes longer, its too small, if it takes a lot less time its too big.

Let me know if this helps and if you have any other questions more pertinent to triathlete swimming because I have a friend I can ask who is a pro-triathlete.

-Michele