I am not aware of any jurisdiction which regulates the size of a flashlight carried. After all, you can carry your aluminum softball bat in your car, can't you?
The size is rather regulated by practicality. If you would care to peruse the information at the website I gave above, Carley Lamps, you will see that a point of diminishing returns is reached after 4 cells. In other words, adding more cells yields less and less benefit.
Further, were you ever in dire straits, and needed to use the lite in self defense, you will find the 3D and 4D models better balanced.
If you get out of your car, and may be out of it for some time, and it is cold outside, tuck an extra lite inside your clothing. Voltage drops dramatically with temperature.
I have a friend, Ken Cox, who is a medevac chopper pilot in Oregon. He used to use a 6D cell MagLite, but I fixed him up with a 4D cell with rechargable (NiCd) batteries. Here is what he has to say:
****************************
Subject: Great Flashlight
>Hi, Walt.
>The flashlight works great. We have a new helicopter (same model, Bell
>222) and it came with a flashlight holder in the cockpit. My new
>flashlight fits perfectly, except for the soft handle.
>
>The new flashlight puts out a much higher quality light than my previous
>6 D Cell, and it stays bright over time use. I have only recharged it
>twice, because I thought I ought to and not because of any evidence of
>discharge. The recharge with the outfit you sent went brainlessly.
>
>I use the flashlight regularly, but one night scene call comes to mind.
>We flew to a private home on the south side of Bend, where a relatively
>young, visiting physician had an acute asthma attack. We landed in the
>driveway, with trees, structures and wires on the east and south sides.
>
>I continued to run in the driveway as the nurse and respiratory
>therapist went to the patient and brought him back. As I waited, I held
>the flashlight out the open door and scanned my proposed takeoff route
>for wires. The light really punched out there. Sadly, the patient
>died, but he received the best care possible, at least from the moment
>someone discovered his distress and called for help.
Thanks again, Ken
****************************
So there you have it. When a guy positively, absolutely has to see every wire and cable in the area, this is the way to go.
SurFires and their clones are OK, and about as bright as the 4 D NiCd set up, but they are very fragile. Drop a SurFire, and the lamp may break. Drop a Maglite, and you may scratch the case. SurFires to me are a compact, lightweight emergency source of light. But they are not build for abuse, and also, if you are using it in very tight quarters for a long time, you should be aware that Li cells give off HCN (cyanide gas) during discharge. Hope this helps, Walt