Hello, I purchased an apple watch Ultra with an Ocean band. I purchased the Ocean Band because the webpage was saying it has been designed to be big enough to be used with the wetsuit. I am unable to wear it as it is too small.

Right - was wondering the same thing when I got mine. Apparently, Apple sells a longer second strap to use in those cases. It's $49. A link to the white one:

49mm White Ocean Band Extension - Apple

I tried to get it on over a wetsuit and am very happy to see that there is an extension available from Apple. It’s not just white either I just ordered one in black.

in some countries like usa one can get a full refund no questions asked in a 14 days window

I don't believe apple offer only exchanging parts of what is in the box so you will likely have to return the whole thing if you live in an area where you have that 14 days window

you can try to have apple verify it by using their support Apple Support