List Building is always on the mind of most eMarketers and they ten to strive for building huge lists. I know of one site that is trying to build its list to 1 million subscribers (free mind you). If all you are trying to do is get the word out, then that may be achievable but is it wise just to go for the size?
As with anything in business, it is best to have customers that buy from you and would buy again because you can not only give them service, but you can treat them as individuals by looking after their needs. You are in business to show the customer what is in it for them and you will be spending time making sure they do not go anywhere else.
There comes a time when the list you are building is not just customers that buy. The list becomes riddled with lurkers, curiosity seekers, and your competition. If your list is big, everyone will want to have access to it which may or may not be to your advantage. It is certainly to your advantage if your list is targeted at a particular niche and only people that are interested in the niche are on that list.
The point is that you need to build a list that is targeted to the types of products you sell. If you want become an affiliate with another product, then that product had better fall into the same niche or you will lose your members almost immediately. Post why? Because they trust you and if you offer something other than the reason they signed up that trust disappears.
Only you can decide if you want a large list or a quality list. If they choice were mine, I would go for the quality and continue to build my list slowly. The word gets around quickly in any niche and if what you offer fits, people will opt-in.