# How much of a disadvantage is a small sample size?

I am examining a petition involving all UK constituencies. In this dataset 2 of the 632 constituencies have not participated in the petition - in terms of data quality how does this affect my examination?

I am examing which parts of the UK tend to vote for left/right ideals.

When you use methods like linear regression or logistic regression, $$n=630$$ is a healthy sample size. So no problem with that. What could be a problem is that you have censored data (you don't know about the remining two constituencies).