Construct 2 didn't have a json plugin. I don't know about third party add ons and what they can do, I'm guessing they just didn't include the arraysize expression. Or it might be called something else, would need to check the documentation of the plugin.
You can get a similar value and store it in a variable by using a "for each" at the path of the array, and add one to the variable in the for each event.
If you're doing this in c2, I'd say just use an array, the array.asjson expression and load from json action. It's a little more cumbersome because you can't edit the project file directly in the editor in c2 either, but it should still work. I'll put together another example in c2 when I get a chance, it would double as an example for how to do this with arrays.