Mudguards are all nearly the same thickness. Whether you can get 25s under those guards depends solely on your frame, nothing to do with the width of the guard. Whether they will provide you with adequate protection from spray is another thing.
Cheers. My wheel sits mid-way in the dropout, plus I run slicks which seems to direct all the water to the centre of the tyre anyway, or at least, that's how it looks.
With regards to PDW mudguards, has anyone tried to fit x25 tyres under the x23 width fenders, or is there absolutely not enough clearance?