I have a road bike that I use for commuting but I know naff all about them really.

I know I need to try to measure but I was wondering what’s the maximum width most road bikes would allow (so I have somewhere to start)?

It’s a cheapo Felt alloy thing but has racey geometry rather than relaxed.

Link to bike

I think the current tyres are 25mm and I’m looking for something fat, a bit knobbly and puncture resistant for a semi off road ride.

Ta

I think you’d be lucky to have clearance for anything more than 25mm on that. At that kind of tyre width, knobblies won’t help much anyway. You’re probably best to look for puncture protection and run a lower pressure.
I’ve got panaracer gravel king tyres in 28mm and have been surprised at how well they ride off road in the dry. They do a 26mm version I think. Could be worth a try.

25mm

They are right.
Unless you want to ride off camber, grassy, slippery stuff you don’t need knobblies anyway. Just eye up the gap left between the current tyre and the frame and buy a puncture resistant road tyre. Bear in mind that all tyres come up differently and wider tyres on narrow rims will be light bulb shaped and thus unpredictable to estimate accurately.
Short answer? You’ll have to suck it and see.

Hmm ok thanks peeps.

Just not sure …