Size matters in that Virgin prefers you to have 'commercial jet' time:
Originally Posted by Virgin website
We are currently recruiting and ask that prospective candidates have a minimum of 2500 hours total time - if coming from a commercial background this should also include at least 1500 hours commercial jet experience.
As you say, A340 or B744 time is ideal, but many Virgin recruits don't have such types on their licence. Many of our guys are from the B737/A320, and there are a few from the military and other less common backgrounds. I can't think of anyone who's recently joined with 'only' BAe146/RJ experience, but I won't hold myself hostage to fortune by saying categorically that it hasn't happened!
Basically, the core VS applicant has around 4000 or more hours, with 2500 or more on jets of B737 size or larger. There are exceptions, but they needed an extra dose of luck!
Scroggs