ER1C said:

Thanks folks, Its good to know.. If I go into a muddy field now "un chewed up" I wont get out.

So 265 75is wider and more aggressive than 235/85 I cant see anywhere where they do them with wheels built in as I wouldn't have them on all the time.

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I wouldn't hold out too much hope if it is wet grass over saturated clay, irrespective of tyres. Mine wouldn't make much progress uphill in such conditions, if I do need to go out in the fields when it is saturated in the winter I will try and enter and leave the field at places with a downhill or flat route between them. In an emergency, I would take the tractor, it would make progress, but the ground damage is fearsome.
Except in an emergency, I wait until the ground is drier.

I think of tread pattern as a different issue to tyre size, it is possible to get smallish sizes with quite aggressive tread. But I would be wary of too wide, they tend to float or ski, instead of cutting in. I like narrow, but with a good tread on the sides, cuts a rut, stays in it, and drives the lugs on the shoulder through the mud.
7.5 or 9 inch, 205, 225,235, would be the main widths I would consider for a Ninety. Some prefer different!

ER1C said:

Sold... I can always knock the pressure right down to 12 or so and get more than that width anyway.

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I wouldn't go any lower than 16 psi without bead lockers. Pain in the butt if they pop off in the field, or turn on the rim.