Fixed size with Theme Twenty Thirteen and Twenty Twelf

Hello Experts,

I am trying to fix the sidebars at two websites of me.
Both of them show a shifted version on the mobile phone or small screens.
Probably someone know how I can modify these websites:
– https://www.bestaltmed.at/speed_reading
– https://www.bestaltmed.at/forum

They should get fixed sizes instead of fluid one.
Thanks in advance for everybody who can help.

Best regards,
Chris