I am using a gallery format and the font for the captions at the bottom of the image is tiny. I can't work out how to change this! 

Can you add the link to the page?

15 hours ago, sorca_marian said:

Can you add the link to the page?

https://www.buscentreofexcellence.org.uk/test/concfares

Although I have used it on a few pages

You can adjust the pixel size to what size you want it.
You need to add the below code in Page settings -> Advanced -> Page header code injection 

 

<style>
.gallery-caption-content {
    font-size : 20px!important;
}
</style>