Here is something I did before finishing sleeves I thought would be too tight, or too long:
Take the BACK (I hope you've finished it first): Use safety pins to mark out the number of stitches (widthwise) that will reflect the number of stitches of the sleeve at the elbows, mid-bicep, and top of sleeves...moving up the right number of rows inbetween each set of pins.
Now using those markers (6 in all in 3 locations)...make a "mock sleeve" by using a basting stitch...or just safety pin it together to make a mock sleeve. Then put your arm into your fake sleeve. See how it fits.
If it is too tight...un-baste or un-pin it...and move the safety pins on each end, moving them maybe 2 stitches each. You will be making the sleeve 4 stitches wider in each location. Then "seam" it together again, using a basting stitch or just pinning it. Don't use straight pins. You might stab yourself.
The try it on again.
If it now fits good, you have your work order cut out for you.
You can effectively now add extra increases at each end prior to the elbow, then prior to mid-bicep, then prior to the top of the sleeve. I say "prior" because by the time you are AT the elbow...these stitches must in play.
You can remove the extra stitches eventually by K2Tog evenly and gradually across certain rows during the sleeve cap shaping region.
This has worked for me. I'm certain there are other methods, maybe easier. But I like making a mock sleeve using the BACK because it's already knit up.