Hey Annmarie, What's the concern with having a segment of that size?
In some cases it may be too large, but in others it would make sense (assuming the criteria makes sense).
The more you open up the targeting, the less precise the overall list is, and you will see decreasing marginal returns. For example, if you target market is 35-44 years old, and you open it up to 35-60 year olds, then you may see more sales, but those over 45 won't convert as well - assuming your marketing assumptions are sound.
However, if you are prepared to trade unsubscribes for some incremental revenue, then it might make sense for you. It really depends a lot on your email program and what you are trying to achieve.
My experience is that emailable is 15-20%, which means they are not invalid, blacklisted, hard bounced, suspended, or unsubscribed. I'm not sure that's the definition Marketo uses.
My experience is that the Active in Past 90 days AND Emailable is anywhere from 3-18% of the database. Ideally, your targets are in that group, while the wider could be Active in Past 180 days.