is it possible to save files on the device from a WebApp? If it is so, which size does it have?
So hope that there is help for me.
Thanx newone
Nope, unless the user does it, such as the Save Image option.
Mobile Safari supports a number of offline storage mechanisms, but none for files per se.
Use localstorage or HTML5 SQL.
The limit for iPhone is 5MB with option to expand 5MB per time, if you are using HTML5 SQL. (See this answer.). So you are a bit constrained.
See the Safari documentation here.