Guess of real array size in storage.

First of all it probably can save some memory if instead of array of fixed size arrays you keep everything in 1 array and just do the math to index each element.

Secondly it's not clear whether you are concerned about the file size or the memory it occupies when you load it, but probably in memory you'll win even more by flattening the nested arrays

To answer your second question first. In the API info it is written that there is a limit on the size of the Object Store that can vary between devices. Also, values are limited to 32 KB. So I guess the file size.

To explain it a bit better I need to keep track of when the data(time in each HR zone) was created (the day). So if the activity data it's older than 14 days I can remove it. 

I could store it as one array where every multiple of 6 starts a new stored activity.

I also could store the activity data separately with a dynamic key name. And keep track of all the keys. In a key Array.

Or the way I do it now.

But to make a good choice I need a good estimate of the limit of the Object Store. And how to estimate the size of an individual array of arrays like have at the moment.

For example if it is possible to store 100 sub arrays before I would reach 32kb. I have nothing to worry about and that would make life easier (no changes). But if it's more like 10 than that it's a problem.

Why don't you try it? Fill the array with some data, save, read back. If it works then try it for 28 days, etc... You'll find how many days you can save

I'll do that if no one has an idea. Also working on som other issues at the moment. 

I did run the following test (in the startup of my app):

var rray = [2017250,1345, 4365, 345, 34534,1345];
var testArray = [];
for (var i = 0; i < 100; ++i) {
testArray.add(rray);
}
Storage.setValue("testArray", testArray);
var test = Application.Storage.getValue("testArray");
System.print(test);

This gave an error in the print test Line (even the sim gives that)

So you suggest to do the test again but than use a dynamic filled array. For example [2*i,52+i, .....]  and so on.

doesn't have to be dynamic, just don't use the same array referenced 100 times, but instead create 100 arrays:

var testArray = [];
for (var i = 0; i < 100; ++i) {
   var rray = [2017250,1345, 4365, 345, 34534,1345];
   testArray.add(rray);
}

i did it now lik this:

var testArray = [];
for (var i = 0; i < 50; ++i) {
testArray.add([2017250+i,i, 2*i, 3*i, 345+i,13+2*i]);
}
Storage.setValue("testArray", testArray);
var test = Application.Storage.getValue("testArray");
System.print(test[23]);

This worked without a problem. at 80 and 100 it gave an error in the simulator. so I went for 50. I think 50 activities in 14 days should be safe enough don't you think.

Sounds enough. Especially that probably you could even go higher, 'cause you don't seem to have a problem with storing or reading it but only printing which you won't do in the real app.

You could even do this:
for (i = 0; i<100; i++) {print(test[i]);}

this way you can see if the values you read are correct. IMHO they will be.