If I buy a digital book in EPUB size from a web site, how I may read it on my IPad? Specify that I didn't buy it from Apple Store.

Look for an app that reads EPUB files then use the file transfer protocol, of that app, in iTunes, to get your book into that app so you can read it.

I'd search for EPUB Readers in the app store, look over reviews and find one that looks good to you.

First, epub has nothing to do with size, it is a file format that is popularly used for ebooks, but ebooks published in epub file formats vary greatly in file size.

It will depend on where you buy it. Many books are DRM locked (Digital Rights Management, a software lock). If so, in order to load it into another companies reader or reader software, you need to use something like Adobe Digital Editions to "authorize" the book on the device. Apple's iBook software does not work with ADE.

If the book was DRM free, you should be able to read it in any ereader or ereader program that supports epub format.

If the book is DRM locked to B&N, Amazon, Kobo or whichever seller you shopped from, then the simples thing is to download their free app for the iPad (they all make one, in fact they all make free apps for iPad, Android, Mac OS X and Windows).

So the short answer is yes, you should be able to read any epub ebook file on an iPad, although not necessarily in iBook.

So, books from

Amazon - get the Kindle reader app

B&N - get the Nook reader app

Kobo - get the Kobo reader app

non-DRM epubs can be read in iBook, Nook, Kobo, or Google Play

Whether you can read it on the iPad will depend on whether it is digital rights management free or not - if it has DRM on it then you will probably only be able to read it in the store's own app and/or e-reader (the store's site where you bought it from should say where it can be read). If it is DRM-free then you can read it in the iBooks app on the iPad : add it to your computer's iTunes via File > Add To Library, and then sync it to the iPad via the device's Books tab.

Dependng on what country that you are in you may also have access to other ebook apps which support DRM-free epub books - how you then get the book into the app will depend upon what the app supports e.g. file sharing, transfer via your wifi network.