Change text content width

I deleted a couple of side widgets from my website and the text width went full length and looked messy

I tried changing the text width by adding

.single-content {
max-width: 600px;
margin: 0 auto;
}

to my custom CSS in my Theme

Now the text content has gone too small and it wont let me change it back again. I’m very unfamiliar with CSS so don’t know what to do now. I’d also like to justify the text so it looks neater

Can you Help?

My website is https://www.vitalitycolonics.co.uk

The page I need help with: [log in to see the link]