Stuart
Far more important to worry about jaw size than exact diameter.
Historically 4 jaw chucks, especially in the larger sizes, tend to come in heavy duty and light duty types. Heavy duty types tend to have jaw sizes approaching those of next size up three jaw and light duty ones have jaws more like next size down threes. Heavy duty types have significantly bigger and heavier bodies too.
Heavy duty ones hold workpieces more securely but the bigger jaws can seriously get in the way on smaller jobs. Although the heavy duty / light duty split isn't so specific with chucks of the size you are considering different makes and models have considerable variation. For our sort of work a light duty chuck with its smaller jaws and less massive body is generally better so, other things being equal, go for the one with the samllest jaws.
Some of the inexpensive imports have been terribly cumbersome. Objectively, on a smaller machine like yours, if a job needs heavy duty holding power its better done firmly strapped down to a faceplate.
I have both for my Smart & Brown 1024, admittedly a larger lathe than yours. The heavy duty one comes out about once a decade!
Clive