Hello! :)
I'm really sorry but I'm a terrible person to ask about anything related to printing because I don't really do it myself! (It's actually something I should really learn at some point..😅)
If you want to sell prints online from an on-demand service (Inprnt, Redbubble, Society6, etc) you can find on their websites what are their minimum file requirements for printing. That should give you some solid pointers.
Friendly reminder to never post your full-resolution images online, especially not with the genA/I ghouls around.
That said, and this is a generalisation so take it with a grain of salt, but something like 2480 x 3508 pixels, 300 dpi (resolution) should serve you relatively well, I think. It's basically an A4 (don't quote me on that). They're more or less the dimensions I use on my personal work.