button too wide on mobile

Make the button smaller ? For mobile there is an option for responsive screens in the button editor.

Thanks.

Also, how do I make it one sizze on the sidebar, and bigger on another page? Ways to help the scheme – The Pay it Forward Voucher Scheme (pifgiftvouchers.org)

You could use width 100% for the button and then leverage your theme to add blocks / containers with the required width to contain the buttons inside of it.