If you play on a 4K TV and you're sitting more than a couple of meters away, chances are you have to adjust your DPI (Windows scaling) up from the standard 100%. Windows recommends 300% by default when a 4K TV or monitor is connected. Now, the Steam client itself has little issue with this, and scales as perfect as one could expect - but the Big Picture overlay does not.

I've found that depending on the game and the in-game resolution, that the overlay will either be really tiny and stuck in the upper left corner, or really large and doesn't even fit the screen. This makes both chatting with friends and changing controller configurations impossible in BP mode, because you can neither read anything or *see* which setting/button you're editing. These issues also occur regardless of what your DPI is, as long as it's above 100%. I've personally tested with 125, 175, 200, 225, and 250 as well and it's still the same. To a lesser degree with lower DPIs, but still present.

Setting the DPI to default 100 and instead increasing Windows font scaling avoids the overlay problem entirely, but instead causes issues with Windows explorer, as some of the system fonts do not increase in size. There's also the matter that everything looks "off" when the UI itself is tiny, but the fonts are large (makes me feel like a really old librarian; my eyesight is not THAT bad). Now, some people have managed to somewhat bypass the issue by changing DPI compatibility in steam.exe and gameoverlayui.exe to "application" - but it doesn't appear to be a fix for everyone. Certainly not for me.
Changing your system resoluton before every game is also not a viable fix, because who has time for that. It also doesn't look as nice when you're running 1440p on a 4K TV or monitor; this is just an objective truth.

One would think the solution is as simple as not using BP, but that's a major downside when using a Steam controller, as the key feature w/ SC is that you can swap controls in-game and configure everything to your liking. This is far, far easier when you can close the layout and try the controls and resume making changes instantly if something's not right.

I've been in contact with Steam support about this issue before, but was given the most generic solution ever: "set your DPI to 100%". This seems crazy to me, as 4k TVs and monitors are all too common today, and not everyone is sitting on a desk immediately in front of the screen. Other game launchers' overlays scale fine with high DPI settings, but the biggest company of them all can't seem to do this. And I think that's... Curious. 4K players are very far from the majority of Steam's userbase, but the numbers are steadily increasing. This also seems like such an easy problem to fix if they'd just bother, but they don't. It's pure lazyness.