It's best to keep app sizes as small as possible to preserve space on people's devices. All app projects have a 50MB limit, and free Thunkable accounts have a 200 MB total storage limit. If you need more storage fo your account then you can upgrade to PRO to get 1 GB of space for your projects.
You can view a project's app size on the project's details page:
The main contributor to an app's size are files (also known as assets). To add or delete files, click on the Assets button in the side bar. Then click the "+" icon to manage all the files in your project.
You can view your total account storage size in your account settings.
Free Thunkable accounts have 200MB of storage. PRO Thunkable accounts have 1GB of storage.
Read more about Thunkable PRO here.
If you are near the limit, you may be able to create additional storage by removing projects that you no longer need like an old version or copy of a project.