Check out the tyre size calculator in the Motors, 3 wheels, trike tech section of the main site.
tyre size calculatorIt'll help you decide what size you need. The profile of the tyre is a percentage of it's width -so the height of a 70 profile tyre is 70 percent of it's width. Therefore, 2 tyres, the same profile, but different widths, will be different heights.
As a rough guide, we've found an overall wheel/tyre height of around 24 inches gives good acceleration in the first 3 gears, with 4th gear becoming a sort of overdrive for motorway cruising. From 0 - 60, our trikes will actually outrun a lot of road cars. They'll cruise at 70 -75 quite happily & slowly wind up to a top speed of around 85 -90 if you have enough road, but are happiest blatting around A roads at about 60 - 65.