How I know how many times my second loops so I can set an ECX count? help me fix this code (reverse string)

So for this code I need to reverse a string in assembly language, but I need to set an ECX count for my second loop L2, but I don't know how many times I need to se it, so can someone help fix my code?

INCLUDE Irvine32.inc

.data
source BYTE "CIS 335/535 is a great course" ,0
revstr BYTE SIZEOF source DUP(0)
revwrd BYTE SIZEOF source DUP(0)
ecxbkkp DWORD ?
strSize = ($ - source) - 1

.code
 main PROC
 mov ecx, SIZEOF source
 L1:

 movzx eax, source[esi] ; get character
push eax ; push on stack
inc esi

 Loop L1
 mov ecx,strSize ; Pop the name from the stack, in reverse,
 mov esi,0 ; and store in the StringReversed array.
 L2:

 pop eax ; get character
 mov revstr[esi],al ; store in string
 inc esi
 Loop L2
 mov edx, OFFSET revstr ; "Entered: "
  call WriteString

 mov edx, OFFSET source ; User Input
 call WriteString
call CrLf

 mov edx, OFFSET revwrd ; "Reversed: "
call WriteString
mov edx,OFFSET revstr ; Display the reversed string.
 call WriteString
call Crlf

exit
main ENDP
END main