My goodness! You asked the best question! I put this off for a day to think of how best to answer it, and also because this is an excellent blog topic for my Art Journal so if I answer it well I can turn it into a post. (multitasking - yea!)
First I'd like to recommend some light reading of 2 of my older posts:
These talk a bit about painting frequently, how I started doing cows and painting small. But to specifically answer your question the best way is to make a list. There are so many excellent reasons for painting small that even the list has lists. The reasons spawn more reasons...
- Small is cheap.
- Small is fast.
- Small is small.
- Small uses less materials so is more economical ("cheap" #1).
- Because it's more economical you can charge less ("cheap" #2).
- The low prices attract more buyers.
- Acquiring more buyers grows your client base, and your email list.
- You can connect with your new clients on FB and potentially attract all of their friends.
- Because small paintings cost less, some clients buy several at a time.
- Some of the small-buying clients will also like to buy your large work.
- Because small is fast, they take less of your valuable time, so are more economical ("cheap #1). Etc, etc.
- Because small is small, mistakes are less stressful.
- Because small is fast, mistakes are quicker to fix.
- Because small is fast, failed paintings are easier to shrug off, and quicker to learn from.
- Because small is fast, you can paint more of them.
- The more you paint, the better you get!! (This should be Number ONE!!)
- As your work improves, more people like it and want to buy it.
- (And because the small ones are cheap, they can!)
- As your work improves, failed paintings are less frequent.
- Many art-buyers have no room on their walls for large work.
- Many buyers (at fairs and galleries) are traveling and want something they can carry.
- The more you paint, the more paintings you have.
- The more paintings you have, the more frequently you can post on social media.
- The more paintings you have, the more paintings you can sell.
- Painting small makes it easier to paint every day.
- Painting small is addictive.
All of the items on this list are true and from my own experience. None are made up or borrowed from anyone else, but you might hear other artists have the same advice because they've learned it too!
I could keep adding on to some of these - especially about the social media! But that one might need it's own post...
Let me know if this is helpful! ;)