Skin mark

About 6 months ago we noticed what looked like a skin tag. Noticed today that it is much larger. Please advise as I am nervous. 

Hello, this definitely merits a visit to the vets.  If you go to the vet help direct Blog (there are links around the site) and type 'lump' into the search bar, you will find a multitude of articles to help with this, including one that I wrote a few years ago now.  Essentially, one cannot identify a lump by looking at it and your vet may well want to remove and / or sample this one.  I can't see where this is exactly, but mammary cancers are very common in dogs.

(and very difficult to differentiate the benign from the malignant by eye).

You are right not to ignore it.