Need some help with tires

Well, a 7" rim is 177.8 mm wide - just multiply 7 x 25.4 (25.4 mm in an inch).

If then, for example, you want to shoot for an 8" (203.2 mm) wide tire we're probably
talking about a 205 (195, 215 and 225 might be alternatives - just divide by 25.4 for inches).

It's that first number of a metric sized tire that is the width of the widest part when mounted
on the median width recommended rim and inflated to the recommended pressure (be it
measured at the tread or the sidewall bulge - whichever part of it is the widest).

The second number of a metric tire size (50-60-70 etc) is the "aspect ratio" - the height of one
inflated sidewall compared to the tread width (as a percentage) and since we already know the
diameter and width of the rim you are using and I'm guessing that you want about a 26" tall tire
(if not, we can plug in other values) then we can use all of these numbers, plugged into a formula,
to determine the aspect ratio that will give you the desired overall tire diameter.

Tire height formula example:

205 (the width in mm) x .55 (the "aspect ratio") x 2 (for the 2 sidewalls) divided by 25.4
(converts to inches) +17 (the rim diameter in inches) = the overall tire height in inches.

This 205/55-17 tire would be 25.88" tall.

Using the same formula, a 225/50-17 would be 25.86" tall and 20 mm (about 0.8") wider.

You see, on the second tire I increased the width but then brought the height back down to
the first tire's by decreasing the aspect ratio from 55 to 50 (the sidewall height went from
being 55% of the tread width to 50%).

If you have some different widths and heights in mind, give it a try or post them here.