Regarding your queries:
You are able to store it in your browser safely unless and until you do not clear the cache of your browser or do not upgrade your browser. If you wish to do these acts, it is always recommended to submit the data to your KoBoToolbox server.
I am not very sure on this but it should be able to hold an ample amount of text data. However, there may be safe storage issue if you have a lot of images, videos or audios as these files consumes a lot of space. It is thus always recommended to upload all the submissions to your KoBoToolbox server once an internet is accessible to your device.
Always recommend users to make a demo test before starting the main survey project.
As said earlier, you are able to store data it in your browser safely unless and until you do not clear the cache of your browser or do not upgrade your browser. If you wish to do these acts, it is always recommended to submit the data to your KoBoToolbox server.
However, it’s always advised not to keep your data in the browser (for a long time) once you have completed your data collection for the project. It’s good to sync it with the KoBoToolbox server.
Maybe this support article Collecting Data through Web Forms should be helpful for you to better understand how users could best collect data through web forms (aka Enketo).
