Worth saying plainly: most unisex garments are cut to a men's block. Straight through the body, wider at the shoulder, longer in the sleeve. Calling it unisex describes who it is sold to, not how it was drafted.
What that means in practice
If you usually wear women's sizing, a unisex garment in your equivalent size will be roomier through the waist and hip, longer in the body and sleeve, and wider at the shoulder. None of that is a fault — it is the intended look for a relaxed fit. It is only a problem if you were expecting a shaped garment.
Two workable approaches
Size down one. Gets you closer to a fitted look. Check the shoulder measurement before committing — if the shoulder is already close in your usual size, going down will pull across the back.
Size up and style it. Take the extra volume as intentional and define the waist yourself with a front tuck or a belt. This is generally the better-looking option, and it is what most people are doing when an oversized tee looks deliberate rather than accidental.
What does not change
Shoulder width. You can tuck, belt or layer your way around a loose body, but a shoulder seam halfway down your arm always reads as a garment that does not fit. Use that measurement as the deciding one.
Sleeve length
The most common complaint about unisex tees, and the least fixable. If sleeve length bothers you, check the Dimensions section for sleeve measurement specifically rather than trusting the size letter.
Our unisex range
The unisex collection is the largest part of the shop, which is why this comes up often. Every product page lists actual measurements under Dimensions.
See also UK clothing sizes explained and, for styling around a straight cut, the women's styling guide.
Further reading: Wikipedia: T-shirt