Terry,
The DOT specifications I came across in an SAE publication down in my public library, reference section.
It expresses lights both in surface area and light intensity with both maximum and minimum values.
I would have to return to the library and take note of chapter and verse but this info is probably available on the internet somewhere.

My BMW 325i (e36) calls for 32/3 candlepower for the tail lights, being 4 watts for the running part of the double filament (similar to 1157).   The other part of the taillight calls for a 5007 bulb which is 5 watt also, making the total running lights to be 9 watts per side.
My BMW 325e (e30)has lights of similar strength. The running light bulbs are either 5007 or 5008. I will check when I get home.
I cant imagine why your BMW 325 (e30 or e36 ?) would be any different.

Once again I can identify with your desire to have tail lights that are easily seen.   I do too.   However it is my opinion that the running lights should not be as bright as brake lights normally are.
1156 bulbs are brake light bulbs.
Often older vehicles have smaller lights than more modern ones and this is regrettable but making them work as well as they can by cleaning the plastic and the contacts can do a lot.
Have you checked the voltage at your rear lights?
Regards,
Steve