What is the data size requirement for this upgrade?

It is displayed as part of it's page in the App Store. Yosemite is about 5.3 GBs.

You can also download the 10.10.4 Combo Update that is about 2GB, and install that.

OS X Yosemite 10.10.4 Combo Update

Lex

And, you did not consider my reply as helpful?

dinjim wrote:

How do I find the size of an upgrade when announced?

I have limited data allowance per month and do not desire to go to overages.

What version of OS X do you have now?

The 5.6 GB file is for a full install of Yosemite that you'd use to upgrade from an earlier version of OS X (e.g., Mavericks) and is what Kappy mentioned to accomplish an upgrade. As Apple releases updates to an OS X version, the full upgrade installer that Apple offers in the App Store will also be for the latest version.

If you already have Yosemite (your first post says 10.10.3), you only need an update and the single step from 10.10.3 to 10.10.4 can be accomplished with the OS X Yosemite 10.10.4 Update. That's just a bit over 1 GB which may fit within what's left in your data plan.

There is also the Combo updater that LexSchelling mentioned and that's larger because it can update existing Yosemite installations including the very first version of 10.10.

But what you need depends on what OS X version you're starting from.

The Combo update adds what is changed only, not the whole OSX, but only what is changed since the first introduction (i.e. a combination of all later updates).