Possible, the 60 refers to the sidewall height in relation to the width and is shown as a percentage i.e. 60% of the width so the 60 really depends on the width your planning if having, to confuse you they measure the width in mm and the sidewall as a percentage. The 20s normally use 245/45r20 so you can already imagine what a 60 sidewall would look like on a 255, would be huge
Use tire size calculator given above.
Without a lift, the largest diameter I have seen confirmed as fitting is 763mm. I have fitted 255/60R18 instead of the original 235/60R18 and all is fine (tire fitter was dubious!). Any larger and you'll probably scrape.