Type a size like 265/65R17 for full dimensions, then add a second to compare and check if it fits.
265 is the section width in millimetres, 65 is the aspect ratio (sidewall height as a percentage of the width), R means radial construction, and 17 is the rim diameter in inches. So 265/65R17 fits a 17-inch rim, is 265 mm wide and has a sidewall about 172 mm tall.
As a rule of thumb the overall diameter should stay within about 3% of the original, and the rim diameter must match. TyreSpace shows the diameter difference as a percentage and flags it green (within 3%), amber (3–5%) or red (over 5%, or a different rim size).
Keeping the overall diameter within about ±3% of the original keeps your speedometer, odometer, gearing and ABS/traction systems close to their factory calibration. Beyond that those readings and behaviours drift, which is why the verdict turns amber and then red.
Yes. A larger overall diameter covers more distance per revolution, so your speedometer reads slower than you are actually travelling. TyreSpace shows this directly — for example "reads 100, doing 107" — based on the diameter difference between the two sizes.
Often, within limits. Each rim width suits a range of tyre widths, and a wider tyre can foul suspension or bodywork. TyreSpace shows the section-width difference so you can judge the change, but always check the physical clearance on your own vehicle.
Overall diameter is the full height of the inflated tyre; rolling circumference is the distance it travels in one revolution (π × diameter). Together they set your effective gearing and the revolutions-per-kilometre figure TyreSpace calculates.