For our first session we used the (really, quite nice) grid map that comes with the Pathfinder starter kit.  Since we were using the adventure from the kit, the map was pre-printed and awesome and got us going, well, right out of the box... 

At the time I had no intention of building lego dungeons or maps.  I was just thinking of using the lego figures as avatars and using whatever happened to be laying about as monsters.  I started searching about for custom figures and monsters, and ran into Brickquest, and fell madly in love with his dungeons.  Peter (the Brickquest guy) made an actual custom game to play in his lego dungeons, which I wasn't interested in doing, so I quickly realized my dungeon tiles needed to be bigger than his.  (Also, I think some of the pieces he used are no longer in production.)

I decided to use Lego's 10" base plate as my "regular" room size, with smaller plates as connecting hallways.

So setting down with the random lego I'd managed to pick up, I built this, my first dungeon.

I liked it, it was cute... but it wasn't going to work.  

For one, I had a large back wall, to show off my awesome lego building mastery (that's sarcasm), but as soon as one of my players saw it, she pointed out that nobody on that side of the table would be able to see over it.  

For two, and more significantly, it was just too small... our party would barely fit in the first room, let alone our party and enough combatants to have a meaningful fight (and forget flanking!)  

So it came down.  

As an experiment I built the map from another pre-written adventure from Paizo (Something about saving a pair of dwarves, I think it came from a starter kit expansion that I grabbed from their websites).  Using a 4x4 plate as one grid square, I was able to build it to scale, and everything seemed to fit okay.  (Although a giant spider - aka Aragog from one of the Harry Potter sets) is much bigger than 10 feet by 10 feet at this scale, at least if you count his legs...)

Of course compared to the figures, he's definitely lives up the name Giant Spider, right?

When we did actually get together to play, we played at a pub, which I have to admit being nervous about, but (noise issues aside) went pretty well.    I was a little worried we'd get beat up (too many DnD stereotypes in my brain apparently), but everyone seemed to think it was cool.  I do live in Portland though, so your mileage may vary.  We ran through this with lego figures on the bar table, with no grid, and I just drew the map on a napkin.  So we never used that dungeon either, but at least it proved I could build a reasonable sized dungeon.