1. The answer is a bit of both - but size of audience doesn't determine curation in most cases- my first curation came when I had less than 50 followers, and many since, despite not a huge following.

2. Your overall consistency shouldn't mean anything for overall curation - for Medium curators your articles are a drop in an ocean - so whether you write one a day or one a month, they'll continue to have volumes - you have the best shot at curation when you write a great article and follow Medium's Curation guidelines

3. 4-7 min is "considered" to be ideal length but many shorter form articles are curated too - and infact Medium is now even promoting short-form, go to Medium Topics pages and you'll see shorter curated articles too. Poetry for example is always short!

Having said all that - not all good articles are curated, but writing quality articles does enhance your chance of curation. I've written a fair bit about it if you want to read!