In 7.3, there is no longer a property MaxRequestContentLength, as this was a property of the http dispatcher, which no llonger exists in 7.3. Therefore all requests run through the ICM, which in this case takes over the old task.
You have to add a line like
into your profile file (under \usr\sap\<SID>\SYS\profile) of your instance (or into default.pfl).
After that, restart the ICM at least.
You can check if it worked via /sap/admin/public/default.html (user is "webadm", password is the master password), Core System -> Parameters.
Hope it helps