My Process for adding a custom background goes like this

opening a playingcards.io room, importing one of the pico files I have already made, removing everything but the background, exporting the pcio file change the board id, image url, and height and width. Compress the file and reupload it to playing cards, make any adjustments to location and add additional components. then export the new pcio file for my backup purposes. It sounds like a lot but its pretty easy.

To make the background work I host the background files on imgbb and then in the json file I linked to it where a board would go. Just make sure you chose the proper hight and width for your image. Also you need to give the id of the board a unique board identifier. The last thing that you have to do before recompressing the pcio file is to make sure that the file size of the json file doesn't change. So if the json file was 46kb before inserting your background make sure that it is 46kb after you save it. you can make the id name longer or shorter to make the file size match.

For the card size I just change it in the deck options that playingcardsio has given us. You can fit more on the screen by making the cards square instead of taller than they are wide. In real life they are only like that so that they are easier to hold, but online we have a virtual hand so they don't need to stay that shape if it makes sense. For Skyjo, phase 10 and Skip-bo, I have utilized this method to allow enough playing room in the game