35mm is generally what's available. However, it will make your ride a hell of a lot more choppy, and it is advisable to fit a new set of 4 shock absorbers as well.
Also be aware that your more prone to whel bearing failure, steering failure and bad tyre wear than on a standard suspension set-up as the whole suspension/steering geomatry has been thrown out the window.
Also expect traction problems and possible skating.