According to the Royal Shakespeare Company website, "goose" was a slang word for prostitute in Shakespeare's time. Therefore, it is likely, as Shakespeare loved double entendres, that the porter is punning on that meaning of the word "goose" in this line.

A goose was an iron a tailor would use, so the overt or obvious meaning of the line is to invite the tailor in to roast or heat up his iron in the fire. However, the sexual meaning would be the following: come on in tailor, here you can warm up your prostitute, with roasting also having a sexual meaning of warming her through close bodily contact.

In Romeo and Juliet, Romeo uses the word goose in Act II, scene 3 to mean prostitute—so Shakespeare was aware of the slang.

*All information and translations are adaptations taken from The Norton Shakespeare, based on the Oxford Edition.

The information I can find regarding this quote ("Come in tailor. Here you may roast your goose." Lines 12-13), is that the tailor has been sent to the porter because the tailor has not provided enough fabric for a pair of breeches. The comment: "Here you may roast your goose" is interpreted to mean that the tailor can heat his smoothing iron, but also, the word "goose" could refer to "swelling" which would indicate that he is suffering from some kind of venereal (sexually transmitted) disease.