My tyres are due for replacing on the trusty Honda Sonic. What is the biggest size I can get away with?

Many of my farmer neighbours have the nobbly type tread on trial bikes fitted, its pretty rural up here. (Phetchabun).

Current size tyres:

F. 60/100/17

R. 80/90/17

Those in the know, many thanks.

I'd stick with what you have already, that size seems pretty reasonable to me. (out of interest why do you want to put a bigger size on?) 'nobbly' tyres are fine just be careful that they do not catch on mudguard, shocks and swing arm.