How can I decrease screen size because it is so big that I am unable to see any ticks to reconcile.

Welcome to the QuickBooks family, @helenmmcgeorge.

I can show you how to adjust the screen size in QuickBooks Online.

The default view used by the system is set to 100%. I suggest you manually zoom in/out the display screen to either increase or decrease the font size and other options.

To zoom in and out, you can hold the Ctrl key and press the Plus sign (+) or Minus sign (-) on your keyboard simultaneously. 

I've also attached a link below for additional reference about using QuickBooks Online: Help articles.

Please know that I'm just a post away if you have any other questions. Have a great day ahead. Let me know.