When buying a jumpsuit it's best to buy according to your height, this is because our unisex styles are made to have a comfortable fit and it's based on the height in the chart below.
XXS: 158-168cm (5ft1 - 5ft5)
XS: 164-174cm (5ft3 - 5ft6½)
S: 170-180cm (5ft5 - 5ft8)
M: 176-186cm (5ft7 - 6ft)
L: 180-190cm (5ft9 - 6ft2)
XL: 184-195cm (6ft - 6ft4)
As you see it's overlapping, and this is for example if you are 185 cm tall, but if you weigh 75 kg. According to the height you are a size L, but because you're then quite slim, the size M would be a better fit, so you'd be a tall M.
If it's your first time buying, it's maybe good to stick to the smaller of the two sizes you are considering, because most of our first-time customers that return a product do so mostly because it's too large.