If you have the experience there is no enough storage on your device, you can use the USB drive or micro SD card as a extra storage for your device.
After you plug in the USB drive or micro SD card on your device, you can go you the editing screen session.
Click Advanced, then click More
Check "Use external Storage" and select Auto Detect in the "External Storage Path".
After that, OptiSigns app will use the external storage on this device.