If you don't know their size
A mug, a cushion or a canvas removes the problem entirely. All three carry the same artworks as the garments, and none of them need a measurement.
If you do know their size
A hoodie is the safer garment gift — sizing is forgiving and the heavier fabric reads as more of a present than a tee. Check the size guide rather than guessing from another brand.
If you know the artwork but not the person
Start at the artwork page. Every piece we print has one, listing every product it appears on, so you can pick the format that fits the recipient rather than the other way round.
If you have left it late
Everything is printed to order, so the honest answer is a gift card — it arrives instantly and they choose the size themselves.