Sleeve width from your bicep plus wearing ease.
Fitted sleeves use ~1\u20131.5\u2033 ease; relaxed sleeves 2\u20133\u2033.
Sleeve width at the upper arm equals your bicep measurement plus the ease. Ease is the extra room that lets you move and dress comfortably, and without it a sleeve feels tight and can tear at the seams when you reach or bend your arm. Measure around the fullest part of the upper arm with the tape snug but not pulling.
Fitted garments often use about one to two inches of ease at the bicep, while relaxed shirts and jackets use more, and a coat worn over layers needs more again. Knit fabrics with stretch need less ease than woven fabrics, which do not give.
Test the fit by making a quick mock up in cheap fabric, or compare the measurement with a sleeve that already fits you well. Consider how you will use the garment, since activities that involve a lot of arm movement need extra room. Remember also to allow for seam allowances when you cut the pattern.
If the sleeve will be set into an armhole, the cap of the sleeve often has a little extra length that is eased in. Check the pattern instructions, and adjust the width and the cap together so the sleeve hangs without pulling.