How can i tell size of watch?

On the watch, go to Settings>General>About and compare the model number to the following;

A1858 (38mm GPS) A1859 (42mm GPS)

A1860, A1889, A1890 (38mm GPS + Cellular) A1861, A1891, A1892 (42mm GPS + Cellular)

M1 Mac mini, macOS 11.3.1; Watch, series 5 GPS/cellular, watchOS 7.4.1; iPad Pro 2nd Gen, iPadOS 14.5.1; iPad Air 2, iPadOS 14.5.1; iPhone 12 Pro, iOS 14.5.1; TV 4th Gen, tvOS 14.5  HomePods, audioOS 14.5 and Airport Express w/ Airplay 2

You can also look on the back of the watch.