Despite having this issue for a while now, I solve it within 5 minutes of posting this query! It seems that PickAtlas runs on a fixed grid window, so needs an adequate resolution. 

If anyone is experiencing the same problem, I fixed mine by altering the resolution on my macbook. I was using a 2015 12" Macbook on the standard resolution settings ('default'; looks like 1280 x 800). I switched to the 'more space' setting which looks like 1440 x 900.

To do this, take these steps;

System Preferences > Display > Resolution > Scaled > More Space*

For resolution, there are the options of default display and scaled display. Check scaled display, and click the 'more space option' (looks like 1440 x 900). *Note the actual resolution scales may vary per laptop, but just aim for a general increase in the resolution numbers, this should work for windows etc. 

Hope this helps someone!