How to change the size of my logo and text – WordPress theme

Hi all,
I am looking for help to reduce the dimensions of my logo in the header (I have tried changing the dimensions of the image, but it doesn’t affect the size of the logo on my site).
Also, I would like to know how I can change the text size of the body text throughout the website rather than changing manually.
URL: http://www.fona.org.au
Thanks so much for your help!