I know that Sirius A and B orbit each other. I'd like to ask how large is their relative apparent motion in arc seconds, seen from Earth?
From Sirius A to Sirius B on the sky is around 3 arcsec on average.
The separation between Sirius A and Sirius B actually varies from 3 to 11 arcseconds b/c of their orbital motion over a long period of time (Here).
I believe you're referring to that. There's a lot of information (and even some graphs and diagrams) here: Here. The information on that website is sort of vague, but I think it'll be helpful in visualization.
Hope that helps!