Thanks for contacting us. To maximize the resolution of your collage you can:
1. Change your Image Quality setting to Archival by clicking Hi Username on the top right toolbar
2. When creating your collage, change the Quality on the top toolbar to High
This will allow collages at up to 3600×3600 which 24″x24″ at 150DPI.
To edit your current collage, open it up from the Library > History tab, and then click Undo until back in Collage mode. You can then change the Quality to High.
Let us know if you have any other questions.