Not sure, but.....
IF right clicking the individual tile and selecting an alternate size doesn't do it for you, then.....
THE first place I would look is ....
Right click a blank space on the desktop and click Display settings
On the slider labeled "Change the size of text, apps, and other items:" try shifting that left or right and see if you can get the desired results.
Otherwise it may be linked to your display resolution, so....
Click the "Advance display settings" link on that page and in the Box labeled "Resolution" try sampling a resolution a bit lower to see if that produces a better experience.
IF your problem is only impacting the "Tiles" in the Start Menu, then you can probably ignore this post, as the settings I've suggested are global in nature and effect most everything you see on your screen.
There may be a registry key somewhere that dictates the actual Tile size, but I have no idea what or where it might be.
