Watch Band too small

XL/XXL/10.5” appears to be the largest band size commonly available.

You’re probably going to need a custom band.

There are also adapters that fit a pin-based band you might already have onto the Apple mounting. VICTHY offers one.

There are also folks that hand-make custom bands and straps from various materials, and that may be amenable to your requirements. The Strap Smith appears to be one.

Check Apple Online here: Buy Apple Watch Series 5 Bands - Apple

FYI: Those Series 5 bands also can be use for your Watch Series 4

You need to know the Watch case size.

Go and buy a band that fits.

FYI: Apple does not sell Watch Series 4 since they released Series 3 and 5.

As stated Apple do not sell Watch Series 4 anymore, so you have to return it back to wherever you bought it from

and buy directly from Apple.

Sorry this a technical forum form user to user,

you need to use Contact Support at the top of this page.