[Tunicates]

Weeeerrllllllll, you havnt told us what you weigh yet......but I'll guess in the 16 stone region.

The frame should certainly be strong enough, but it might be a little more flexy (soaking up and wasting some of the energy you put into pedalling) than it would for me - same hight but only 12 stone.

Frame size - you want a 21" frame. THhs is measured from the middle of the bearing case (bottom bracket) that the pedal arms (cranks) are attached to - up to the top of the frame where the seat-post starts sticking out. OR maybe 1/2 an inch below it. Its different for different brands.

If you do go for a 2nd hand Kona, these are only measured S/M/L/XL I would say you would fit an L. This may be true for some other brands also.

Mountain bikes have 26" wheels. The width of their tyre are measured in inches. 26x 1.3 is as thin as you would want on a road optimised tyre and they'll go up through 26x1.5, 26x1.75, to 26x2 which is as fat as a fat thing for purely offroad use.

Road bikes and hybrids have a wheel size called 700c (used to be 27", but not quite the same) and the tyre widths are measured in nominal mm. They are only a rough guide and dont actually measure up to a ruler or anything.
700x19 is for racing bikes only and you dont want them. 700x25 is for whippets that want some comfort. 700x28 is for fast commuters that have to carry a bit of luggage. 700x32 the same but a bit more luggage.
What you should be looking for is a bike that will take upto 700x45 AND a set of mudguards. Most hybrids of your price range should be ok for that these days tbh.
Best make of tyre in this size at the moment is Bontrager. Specialised Armadillo's are another highly rated for fat bastard heavy use. Pretty well bullet proof. They dont need to be really nobbly either for the offroad stuff.
A good solid central tread that will be fast onthe road sections and a few knobbles on the edges would be ideal.

Btw, that bikesjop finder only works with a post-code. The first three bits would help ;-)

If you are more of a man than my guess then I might reccommend something a bit more burly.....