For brand specific sizing, use the "Size & Fitting Guide" link found on every product page.
Often the fitting of outdoor clothing and footwear will vary from what you are used to, so here we hope to let you know what sizing and fit to go for depending on the brand and activity.
Anything designed for climbing or scrambling over terrain and using your hands is likely to have a longer sleeve, so it might feel more generous in the arms than you are used to.
The armpits might also feel more ample.
Both of these things are so that when you put your arms up or out, the sleeves won't ride halfway up your arms but will remain in place.
For casual wear, and when you arms are at your side, you may feel that the garment is too big, but this isn't the case.