Yeah, I generally resize in Photoshop because I like to minimize the RAM and MB usage, especially since in this case we're talking about a lot of largish map tiles.
But if I HAD to do it all in BRPG, I would:
1) Create a multi-token object containing all the tiles.
2) Use the "resizeall" command in the chat window to set the exact size I need for proper alignment with the grid.
3) Break apart the unit into many distinct objects.
4) Lay out all tiles end to end to create a complete square or rectangular map.
5) Optional: If I wanted to randomize the map setup, I would select all the tiles and then use the "Shuffle Locations" command. Bam! Instant random map.
Please help spread the word about BRPG and BGE, and never hesitate to tell me how I can make them better suit your gaming needs.