Looking for new stem. What size?

I have an early '04 slayer 18" with what appears to be '03 specs. I have added a Thomson post and carbon cx riser bars and now I am looking to add a Thomson stem - I just don't know what size the bike came with. Any ideas>

Also, last month I changed out the POS bottom bracket, added new small and middle chainrings, and SRAM chain and cassette. Rides smoothly again.

Candy SLs are the sh!t. . . worth every penny. SPDs were ok in their day but 'beaters are so much nicer.