What size is my wheel?

Probably. It’s not like the radius would be 1” or 2”, right? RIGHT??! kidding

The tire should have markings. Something like “26×2.1” or “700×28”.

@jaytkay
Thanks.

Tires for road and hybrid bikes are either 27” or more commonly 700c, a metric size that is around 1/8” smaller in diameter than a 27”. However, they take the same inner tubes (There is also a 650c size that is closer to 26”, but you find those only on specialty bikes made for time trials and triathlons). Mountain bike and cruiser tires are mostly 26”, although 29” tires have come into vogue for high-end mountain bikes recently.

You can use a smaller inner tube than the tire calls for, within limits, e.g., if you have a 26×1.75 tire, you can get away with using a tube made for 26×1.25 to 26×1.5 tires. You don’t want to stuff a tube made for a 26×2.0 tire into a 26×1.5, though, because there will be wrinkles in it that will chafe and eventually rupture.