INT 15 - TopView - SEND MESSAGE - "LEN" - GET OBJECT LENGTH
AH = 12h
BH = 09h
BL = object
00h handle in DWORD on top of stack
window: get chars/line
timer: get 1/100 seconds remaining before timer expires
mailbox: (DV/X) get number of bytes queued to mailbox
01h get number of chars/line in current task's default window
0Ch (DV 2.26+) get chars/line in window owning handle on top of stk
0Dh (DV 2.26+) get chars/line in parent task's window
Return: DWORD on top of stack is length

See also: 1512

1512 - TopView - SEND MESSAGE - "LEN" - GET OBJECT LENGTH