How small can a spiral galaxy be?
The first Google-hits suggest that Segue 2 is the smallest spiral galaxy, but that is actually incorrect since it is the smallest spherical galaxy yet discovered. It has a radius of roughly 34 parsec or 110 light years. The original publication on that topic dates back to 2013, but it was again in the news in April 2020.
Then, there are lecture notes on a course Astronomy 626 Galaxies which shows the following diagram:

Also, the lecture notes claim:

Typical values for the scale length [of spiral glaxies] are: $1 {\rm kpc} &lt; h_R &lt; 10 {\rm kpc}$

I agree that this does not answer your question fully, but I got the impression that the lower bound should be around $1 {\rm kpc}$.
References

https://www.nasa.gov/press/2014/september/hubble-helps-find-smallest-known-galaxy-containing-a-supermassive-black-hole
https://www.treehugger.com/just-how-big-is-the-smallest-galaxy-in-the-universe-4862842
http://www.sci-news.com/astronomy/article01147-segue-2-dwarf-galaxy.html
Evan N. Kirby et. al: Segue 2: The least massive galaxy
https://physics.stackexchange.com/questions/148418/