HOw can I edit Size of my Blog??please answer!!

I know may be this is easiest question but I am not able to find the answer and I am searching for almost 3 hours but still no clue.
I want to edit my template size as it’s smaller.
my blog is at https://www.skincaretreatments.net.au
I just want the content..to appear in wide space rather than current.

thank you in advance if anyone try to help.