I have a Vanmoof VM3 bike that has a coaster rear brake and I want to add a rim brake. What size should I get or do you know one that is compatible to this bike?
If your bike looks like the stock photo, then possibly. However you may need new wheels.
There's a bolt that passes through the fork from front to back, just above the mudguard/fender. This would be where a caliper brake would mount, and the mudguard would be "pinched" under the nut, on the other side of the fork from the brake mech.
You would need a side-pull caliper which has enough "reach" to get out and over the mudguard, and then down to the brake track on your wheel's rim.
This fork would not support a V brake or any other style that bolts onto a pivot below the brake track. A caliper brake like a tektro R559 would likely reach. (not a product rec, just an example of the style)
You'll also need a brake lever for your handlebars, and an inner cable and length of outer. I'd suggest looking for a lever that can clamp around your bars without having to undo the fancy leather-like grips. They may not survive being removed and reinstalled, so a brake lever with a two-part clamp will make it much easier.
However, before you start, look at your front rim and figure out if it has a brake track or not. The brake track would be silver or possibly black, but would generally be flat and just inward of the tyre. This is a rim brake track:
I suspect your bike's rims may lack this brake track because it wasn't designed to need one. If there's no brake track, you need a new rim on your wheel, which may require a new front wheel instead.