Yup, it's near Place Vendome, rue Faubourg st Honoré.
They don't have so much stock actually, you have to place an order most of the time.
I asked today about a St Louis PM and for instance the white would only be available in 1.5 months.
So I guess you could call before you come in Paris. The SAs are so nice in there!
I'm sorry i don't know the Urbain... but here are the dimensions for the different St Louis:
(when I want something I tend to be obsessive so I have all the info about them)
junior: 11.5X8X4
PM: 18.75X10.5X5.5
GM: 23X13X7.5
(inches)
I saw the junior today... it's teeny tiny. Super small.
I tried on the PM, it's about the size of a regular longchamps tote, and I was able to wear it on the shoulder, and I have big arms. Hurrah!
I didn't see the GM but it's so huge supposedly it looks more like an overnighter...
Now I only have to decide which color I should get

I touched the canvas and you can just tell the quality of the craftsmanship... Swoony!