I was resizing some Seiko watches the other day. The most important thing to watch out for when resizing the Seiko monster bracelet is the locking collar and pin that hold the bracelet links together. Seiko use the pin and collar locking system and the collar is plenty small.
The pictures below, don't serve as a step by step guide. Apologies for poor quality of some of the pictures as the background is not the best in the world...
The Seiko Monster SZSC003 Blue Ocean.
Disconnect the bracelet from the clasp.
Mount it on the tool to punch out the pin.
The removed link, pins and locking collars. See how small the locking collars are.
The watch, Seiko SZEN006.
Note the direction to punch out the pins.
Disconnect from the clasp. Note the spring bar. Don't lose this. :)
The link removed. Note the pin and locking collar.
Reconnect the bracelet.
Pin in position.