Like all the rest of the applications it looks awfully small on a hidpi display. Now, it is a wine application and you can change the wine’s DPI setting per WINEPREFIX. However, teamviewer comes with its own, outdated copy of wine that lacks the comfortable winecfg. If you need one, you can just grab a package from an older version of your distribution, chances are it will be compatible.
and then launch winecfg using the teamviewer’s wine home which is (~/.config/teamviewer10 for TV10 and ~/.local/share/teamviewer11 for TV11):
set your DPI in the “Graphics” tab, do OK and enjoy the hidpi mode in teamviewer!