Tell him you want to get him something that he'll wear, but that you don't want to spoil the surprise. Then go and measure him in a bunch of different places. He'll know he'll be getting some sort of item of clothing or apparel, but he won't know whether he's getting shoes, a shirt, a watch, or a ring.

Cut his arm off in his sleep with a machete. While he's rushed to hospital and they stitch his arm back on, sneak out with his watch.

Most watches come with removable links. So they can be retrofitted. And chain style bands are adjustable.

If he's a heavy enough sleeper you can wrap a string around his wrist and mark where it meets to get a close estimate and then talk to a jeweler about what that corresponds to

Get a friendship bracelet kit for funsies. These are the bracelets where each bead is a letter, a heart or a cheap plastic thing. Tell him “I’m making friendship bracelets to put in the stockings for Christmas. I need to practice on you” Make him one. Tell him not to shower with it because the metal is bad quality. You now have his wrist size.

You can also memorize the bracelet order and aragement of the beads and make a copy when he’s gone.

Before we go any further, how tight does your husband wear his watch? To a lot of people it's really particular. If we're guessing use a string and try to match his current habits, if not have a slightly taught string around his wrist and measure.

Or borrow his current watch.

Just take one of his other watches to the shop and ask them for one the same size.

Does he wear a watch usually? You could measure that if so.

Even if it’s not a fixed size, you could tell by the wear on the band what the size is

I did this for my girlfriend, now wife.

Do it while he's sleeping. If he wakes, gaslight gaslight gaslight.

Alternatively, get him a watch and a little tool kit to resize it himself. My smartwatch came with a resizing tool kit and I love that little hammer which I can use to bang out the pins and resize it. Only needed to do it twice till now but so glad I had the option to do it myself rather than having to get a jeweler to do it.

Little hammers are fun!

Ask him to put his fist in your butthole and then take your butthole to the bracelet store and they will measure it

You're kinda screwed if he's an unusually large size. But guys that can just buy a watch and use the band it comes with are easy because adjustable bands adjust for a reason. Be it NATO style, steel links, whatever; only really big or really small wrists will be a problem usually.

So, the easiest way to get close is to use the very shirts he's likely to wear a fancy watch with: dress shirts. They'll be close to the size of his wrists, so you can button one up and measure the inner diameter. Just be aware that that is bigger than a watch band will fit, so that's going to be the upper range of length for the band and watch. If he also wears knit long sleeve shirts you can guesstimate the lower end by using those cuffs, though expect them to be equally approximate since they aren't fitted as well as even cheap dress shirts.

If the gift is a watch with a band already equipped, you aren't going to have to worry unless he's a weird size. But if you're getting a fancy band, be aware that wrist diameter isn't the only measurement needed. You'll need the width of the "lugs" which is the inner width of the prongs that poke out for the band to attach to in men's watches, 16-22 mm is the most common range. 16 is very narrow for most men, with 22 being as big as most casual wear watches should ever go, with 20 being maybe the ideal for 80%. But unless you get an adapter of some kind, it's not really possible to mix and match. You have to stay with the right lug size. The range only matters if you have to guess visually and want to hedge your bets.

You also have to factor in the watch itself. Most bands sold by length are only using the length of the band. So if his wrist is 8 inches around (unlikely, but that's the number at the end of the time on my screen), and the watch is 1.5 inches from lug to lug, you'll need a 6.5 inch long band (or one adjustable to that length). The exception to that is fabric bands that attach by running through the spring posts and under the watch, like NATO bands.

Does he have dress shirts in the closet? The cuff should fasten comfortably around the wrist and not slip over the width of his palm, right?

Does he already have a watch? Does he ever take it off? Check the size that it's set at when he uses it

Does he wear a watch now? Does he ever take it off? Snap a picture top down on its side and put a ruler in the picture for reference. Measure the watch circumference with a piece of string and then flatten it out on a ruler. Keep the string by the bed and wait for him to fall asleep and measure his wrist like that. Suggest using handcuffs in the bedroom and surreptitiously measure the wrist that way.

Somebody else suggested you measure everything about him. Waist, legs, cuffs, etc. That's a good idea as well.

just measure his dick and extrapolate from there