Are you planning to sell that asset? In that case what you suggested is a good idea, putting a watermark that’s hard to remove.
I’m not sure what your question is about? As far as I’m aware, you are free to publish mostly any work you own, including assets. There is not a guarantee on downloads/interest, but you have nothing to lose.
Just make sure you are very clear on what users should expect when downloading your work. For example if they will receive a simple rasterized image (like a .png) or an asset that represents vector graphics (like .svg).
About sizes, I would only suggest when showing off your asset, to show something lower-scaled, as that reduces the chances of someone being able to remove the watermark. Also keep in mind the asset size maximums that Itch imposes.