Could the Roman Empire have gotten bigger?

[Fingolfin_Noldor]

A side track, but is there any sources for Heraclius' great foray into Sassanid territory that led to victory?

Yes, but could you be a little more specific with what you need?

Well, more like the strategies, the battle and the espionage.

[Thanas]

In that case, it would probably be better to read a modern book, since most ancient sources have never been properly translated, or are more myth than legend, like the chronicle of Fredegar. Furthermore, the sources regarding Heraclius are very, very fragmented.

The chronicon paschale, for example, is available in full, but not in english. There is a translation available, but it only covers excerpts iirc.

You surely are better off to read Walter E. Kaegi's book about the Emperor Heraclius, who devotes about 90 pages to the subject at hand, or you can - as always - consult Ostrogorsky. Kaegi's work has its problems (namely, it is a defense of Heraclius), but his work is the newest general history about Heraclius that exists to my knowledge, and he goes into great detail on the subjects you mention.