It’s hard to define if a community is small or not solely by the total of users. What matters is the number of simultaneously active users, 'cause that’s what primarily gives pressure on your server. Codinghorror gave some key numbers to consider in this post.
My forum is on a 4GB instance, along with a self-hosted Matomo analytics. I have nearly six hundred users, but only 13% of them were active in the last month, not to mention how many of them were active at the same time. 
The average memory usage of my machine is 2.6GB.
My guess is that for a such Q&A site with 1000 users, 2gb is a safe choice, if the community is active enough, you may need a 4gb instance at most. And my advice is the same as codinghorror’s, you can upsize a droplet when you find it stressful.
Other than the server charge, the fee should also include the domain fee and SMTP provider charge (which can be reduced to nearly zero if you find a cheap service). Assume that you purchased a domain at $10/year, chose a 2gb 1vCPU server at lightsail ($10), used a free SMTP service, then the minimum donations are $12.