I am not so certain the effect of force size on timed objectives (TO). Rather, it is important to set the TO value in relation to the objective.
One problem, is that TOs are set singular. Once set they apply to all VP hex. So, if you set a bridge as the objective and give it a TO value of 3, all VPs held by the defender will accrue to the defender even if you capture the bridge.
TO requires a more precise design of the scenario with regard to mission and objectives. You must account for force size, but it is not the only determinant. If you have a single VP hex, say to capture bridge, then the TO is easy to manage.
So, a designer has to estimate how long the attacker requires to capture a VP. Therefore, if you say, the attacker must capture six VPs, make sure that those six VP are the overwhelming majority of VPs. Say you have 9 VPs, then make it those six within reasonable possibility by game turns per scenario type: meeting engagement, advance, assault.
Additionally, VP value does not matter in TO calculations, only the value of the TO and the mission type. A hack you may explore, is too set the Turn size higher than you would otherwise without TO.
I have a scenario you could look at:
USMC Alpha Co 1/6 Marjeh 09/2009. You can DL,
Scn595_Alpha_1-6_Marjeh.zip as well.