The supported method to play on a tablet is via the app. Moving things on mobile is a bit hard. On a full OS, you would click and hold the resize icon, then drag left to make the board smaller and right for larger.
You might be able to reset it by clearing the site cookies. You will have to switch back to v3 after.
I use an android tablet to play; on the v2 site the board size on online chess suddenly became really small and I can't change it back. I know about the arrow in the right hand upper corner but it doesn't do anything. This is a shame as I actually am one of the few people who like the new site. On the old site the board size is fine and I can resize it using settings no problem.