You may also see "This export is too large." when you tap Save to Assets.
What to do
Remove layers you no longer need: tap a layer in Layers, then tap Delete.
Use fewer large images in one design.
Tap PNG at the top to download the design to your device. On a phone, it is the download icon.
Split the content across two designs: in Designer, open the ⋯ menu on the design, tap Duplicate, and remove half the layers from each copy.