So, on your rims, is 60mm the inner width or the outer width? Rim marketing usually advertises outer width but the much-narrower inner width is what matters when sizing rims and tires. For examply my MTX39 rims on my Envoy advertise... 39mm width. But the inner width for tire sizing is 30mm.
The WTB gauge chart is a decent reference. I've used it before myself, but the gold standard of matching rim and tire width is the DT Swiss chart. The WTB chart is a bit skewed because they sell tires and as a result the chart has boundaries that ignores the bigger stuff they don't sell - like what
@cluny6 is asking about.
The DT Swiss chart can be faulted a bit because of its corporate origin... expect them to err waaaay over on the side of conservatism. You can fudge their boundaries by quite a bit, but still this chart gives you a really good idea of where the guardrails are. You can also see DT's own marketing prejudice showing as they stop rim width dead at about 70mm but go all the way up to 5" tires.
Ignoring the charts, if you have 60mm rims you can go all the way down to a 2.5" tire, although that is well below what any chart is going to recommend. Lots of people have done that with fat bikes who, for example, are looking for a shorter bike to fit riders of smaller stature (i.e. their wives). I did it once years ago looking for faster rolling wheels and I got that, at the expense of ride comfort. The ride on a 28 mph bike going over potholes was so rough I feared for the frame in the long term... and my spine. That bike had 80mm rims (75 internal) so a 2.5 was a big misfit. I would have never done it except I had seen so many others do it I knew it was safe. You need an oversized tube (undersized for an 80mm rim) to spread the tire and let the bead seat, which it will do happily and without future failure.
So... in this pic these tires are actually 2.35" on 80mm rims. So you see why I say 2.5" on 60mm is perfectly do-able. I believe I was using 3.5" tubes inside yes I really had a sidewall.
On a 60mm rim, riding on the street, a Vee Speedster in 2.8" should be livable and 3.5" will be perfect. But thats a street tire and you want something trailworthy. A knobby in 3.5" should be ideal. So just look for 26" XC tires in 'plus' sizes. Should not be too difficult.
