Actually the 3 dashes in the lower right corner is what you use to change the "resolution of the steam client". This is the same way you would change any other window's resolution that is in windowed mode inside of your OS.
If you want the resolution of what the steam client displays inside of it, your SOL if your on max/full screen. If your looking at the community page use a normal web browser to view it better. The community page just displays html/css/etc stuff you cannot change the physical page because you do not have control over the html display code.
ya the steam window shouldn't be fullscreen always, and yes he said resolution but that is also known as size
in any case if you meant size, do the 3 dashes thing and just change it...if you meant the resolution you change that via your desktop properties
I don't believe you can change the resolution. Sure, you can resize the window but that clearly isn't what you are after. I think the only steam resolutions that can be changed are the in-game resolutions. The steam client its self appears to be capable of only being resized.