You can't change the tile/cover size anymore in the installed games library?

No, it's not possible. The covers on the game tiles are provided by the game publisher, and cannot be changed.

How are you measuring the sizes of your games?

If you go into "My games & apps" and sort by size, it will show the size of each game on its tile, but only the base game; not any DLC.

For example, I have Forza Horizon 5 with both DLC add-ons installed. If I sort by size, it shows the size as 129.1GB, but if I go into "Manage game & add-ons", it shows the size as 159.1GB as it includes the DLC. The difference can be massive, so that can easily account for 100GB of space across several games.

Once I also had an issue where I uninstalled a game, but it did not give me the free space back, so it was acting like the game was still installed, even though it no longer showed in my library. To fix that I did a factory reset, choosing the option to keep my games/apps/DLC installed, and that restored the missing space.

to find the 'ready to install' section

from anywhere, press the Xbox button, then scroll down to 'My games & apps' once that tile is highligted, press the 'A' button, then press 'A' again on the 'See all' tile, once there scroll down to 'Full library' then scroll across to 'All owned games' and
press 'A' when that tile is highlighted then press 'A' on the 'All games' dropdown tile and scroll down so that 'Ready to install' is highlighted, press 'A' again and there you go, there is all your ready to install games.

I hope that helped you.

^^^^Size doesn't matter for something like that.

Disc tray could cover half the screen and advertising have a small tile. It would still be the same amount of scrolling to get from one to the other.

If you own it on disc, just pop the disc in the drive and there should be a tile on the dashboard for the game and you can install it from there.

Otherwise, any 360 games you own digitally that are backwards compatible should show under your Full Library in Games and Apps as ready to install and you can install them there.