I have an 800k json file in my project but Textastic freezes and doesn't return when trying to open it.
It depends on the hardware you are using, but more recent iPads like the 3rd or 4th generation iPad should handle such a file without problems.
But, if the file has some very long lines, this could be problem. Especially, if it is a minified .js file.
Customer support service by UserEcho