I am 6'1" 140 lbs (built like a stick), 31" waist so I was thinking about going with the medium but I see that it recommends a large for someone my height. Also, would this be formal enough for prom?

Tech Specs:

Material:

[Black & Black Stripe] 90% polyester, 10% rayon; [Sand] 100% cotton; [Lining] satin 

Waist:

Belt loops 

Pockets:

[Jacket] 1 breast, 2 hand; [Pants] 2 hand, 1 back 

Recommended Use:

Formal affairs 

Manufacturer Warranty:

30 Days