It’s not well explained under the man but I guess the size and fitness rely on the content of the taskbar. You can specify the number of row with the Row keyword. The color management is achieved via Colorset and FocusColorset keywords. The font is achieved vi Font, SelFont and StatusFont. To give you some hint, here’s my taskbar’s conf:
Style "FvwmTaskBar" NoTitle, BorderWidth 0, HandleWidth 0, Sticky, \
WindowListSkip, CirculateSkip, Colorset 30
DestroyModuleConfig FvwmTaskBar: *
*FvwmTaskBar: Geometry +0+0
*FvwmTaskBar: Rows 1
*FvwmTaskBar: Font "shadow=1 1 se:xft:Blippo:Bold:pixelsize=12"
*FvwmTaskBar: SelFont "shadow=1 1 se:xft:Blippo:Bold:pixelsize=10"
*FvwmTaskBar: StatusFont "shadow=1 1 se:xft:Blippo:Bold:pixelsize=12"
*FvwmTaskBar: Colorset 30
*FvwmTaskBar: FocusColorset 30
*FvwmTaskBar: NoBrightFocus
*FvwmTaskBar: AutoStick
#*FvwmTaskBar: AutoHide 2
*FvwmTaskBar: UseSkipList
*FvwmTaskBar: DeskOnly
#*FvwmTaskBar: UseIconNames
#*FvwmTaskBar: 3DFvwm
*FvwmTaskBar: ClockFormat
*FvwmTaskBar: DateFormat
*FvwmTaskBar: MailBox None
*FvwmTaskBar: StartName
*FvwmTaskBar: StartMenu (Mouse 1) MenuFvwmRoot
*FvwmTaskBar: StartMenu (Mouse 2) MenuFvwmRoot
*FvwmTaskBar: StartIcon $[fvwm_img]/icon/fvwm_small.png
Check my screenshot to see how it renders.