Twenty thirteen editor issues. PLEASE HELP.

Can you post a link to your site with the child theme active?

Its https://www.dub3designs.net

I have (tried to) set the font size as 55px but as you can see, it isn’t working.

This is the only text I see in your child theme’s stylesheet (which is being correctly applied):

body {
font-size: 25px;
}

Did you receive any errors when you edited your child theme’s stylesheet? Are you using a caching plugin or is your host caching your site?

No errors returned and I’m using the SG cache plugin.

I just don’t understand why it doesn’t work

Have you tried emptying your plugin’s cache?

Yes, I think it’s fixed.
My host is SiteGround and they provide a Super Cache plugin. The dynamic part was switched off and now that I’ve switched it on, all changes in the editor are working finally.

Thanks for your guidance.