Sorry, the 9" would refer to the motor diameter. You could go as far as probably an 11" diameter but in that range would be good.
The smaller motors are more able to to be made to spin faster then the bigger motors and so you would have reasonable road speed. For economy of energy you would be looking to gear so that the motor is running faster, near the top end of its speed range. Don;t risk over speeding though as motors tend not to like that.
The idea of using a number of small motors to drive the ring gear isn't new but does throw up some problems.
The first is that the ring gear isn't designed to transmit driving torque much beyond the starting loads of the ICE and also it is not lubricated in any way. It would be difficult to arrange a suitable lubrication system that would work.
That doesn't stop you from using a number of small motors on a large gear but you may find control problems and maybe uneven wear issues with the gear teeth.
Despite your knowledge and experience with small motors it would be easier with one bigger one.
To find out about motors there is a sticky in the motors forum about forklift motors. Have a read through that and see what others are using and asking about.
In power terms an electric motor need not haf anywhere near as much HP as the ICE that you are replacing, maybe only 1/4 or 1/5, or less, of the rated power.
With ICE the power, say 98bhp, is a peak power at one moment and can't be maintained for long. With an electric motor rated at 20kw that would be a continuous power for maybe 60mins. The spec plate would tell you the duty cycle.
But with motor size, try not to be mislead by 'nutters' like me who would be striving to put a 12" monster of a motor into a 500kg trike!