I don't know your particular saw, but:
1. Have you got any sort of caliper (nothing fancy, it doesn't even matter if it can measure accurately)? If yes, just use the "top jaws" (used for measuring inside diameters) to transfer that size to a piece of paper. Then measure that with an ordinary rule or tape;
2. Can you measure (even quite roughly, with a rule) over the outside of the dust outlet. Provided you get "near enough" that should be fine, because;
in either of the above cases, Axminster sell (perhaps I should say "used to sell" - maybe stopped? Dunno, I bought mine some years back) a "rubberised/plasticised" flexible adaptor. About 4 inches OD at one end, steeped at the other, the steps going from about 3.5 inches down to about one inch or so.
If the Axminster adaptor is no longer available, I know there are several similar on the net. But don't know where though, you'll have to search, sorry (or wait until someone else comes along).
Finally "but one", have you got any reasonably thick scrap ply or MDF, and a set of hole saws (cheapo ones will do)? If so make your own adaptor. Use the next smaller hole saw and drill/saw the inside hole first, then sand - or maybe scroll saw - the inside until of the hole until it's a tight fit over the machine outlet. Then use your machine (or a hole saw) to cut the outside diameter to a size suitable to give enough "meat" to give a strong adaptor.
I've made several myself using that method, and it works fine. In one case I stuck two pieces of scrap 19 mm ply face to face together to get enough length on one particular adaptor I made (for my Excali 21 scroll saw actually).
You're aiming for a tight, friction fit, but if you go a bit too far with your sanding, never mind, a length of duct tape does wonders to solve such slips.
Finally, I've never tried this myself but have seen several vids on the net where people have used lengths of plastic plumbing pipe (usually waste pipe?) and even some fittings like 45 degree elbows. Together with careful application of a hot air gun this plastic pipe stuff (PVC? Dunno) can be gently persuaded to fit snugly over all sorts of machine dust outlets.
HTH
