Yes, I had a look at the image when I originally found the FreeBMD entry - it is one of the old difficult to read handwritten registers. I would read the entry as 'SIZER', but the shape of the 'R' could lead you to think there was an 'S' on the end.
Although I did try the new GRO search with the additional 'S', in theory this should only be there if the original registration had the surname "SIZERS" (or appeared to be that). But neither of the surname variations options comes up with a result, and even manual changes like Siger, Seger, Sezer don't help.
I have a burial record for Susanna on 25-10-1860, and she should be around 19 years old. So the district, year and quarter for the FreeBMD record appear to be correct, what I was looking for is the new GRO record as confirmation of her age and that I had the right record.