There were no size designations in the serial numbers for 1988. For the record, that frame was the 1238th FRAME manufactured in 1988. The W indicates that the frame is a Wicked. We started adding the model designation in 1988 since we had Wickeds, Fats, and Team Comps in the lineup. We did not separate the counts of each model in the 1988 serial numbers, that started later (maybe 1990

)
As an example, my personal 18" Wicked is serial number 87777. In 1987, we put the year for the first two digits (87) followed by the number in consecutive order so my bike is the 777th frame constructed in 1987. That year was the first year that Fat City production went over 1000 frames for any year. I believe that convention was used in 1985 and 1986 (I showed up in November 1986 after Gary left to found Merlin)
One thing I wished that we did was to serialize forks but that never happened so it is likely that there are no accurate counts of the number of unicrowns, box crowns, Yo Eddy's, or Big One Inch forks and all the different sizes and configurations (threaded or threadless) that were manufactured during the Somerville era. I have no idea if that sort of thing was tracked in NY or Stowe.
Hope this helps a little
Regards
Scott