I was thinking about making an Ally Spirit steed like a horse, a motorcycle, or maybe even a sportscar. Does anyone know of any rules for increasing an Ally's size? (this is a materializing spirit, not a possession spirit)

Where did you get the idea that an ally spirit had to be small? You can choose the form that it takes and a horse is acceptable. The proviso that I would make would simply that you had to have an ally with a comparable body rating for the form. A horse would probably have a body of about 7 or 8. You could have your ally with a lower body, but it would necessarily be a weak and spindly horse. There's nothing in the rules that says you can't have the form as a horse and minimal scores, but as a GM I would forbid it for reasons of realism. And it will need those stats in order to carry you anyway.

Note that there's nothing that says just because you give your ally the form of a horse, that it can run at horse speeds, however. That goes double for motorbikes. This is a GM tolerance issue. Your GM might assign a karma cost to the speed. There aren't rules for this.

Make sure your ally has Movement power, btw.

I remember some blurb that said a spirit is no bigger than the size of a troll. But yeah if the attributes and the Movement power are the key, I guess there's no reason to pay for size too. Thx.

There was nearly a guideline in Street Magic that a materialized spirit was between 10 and 60 liters per Force point. A riding steed is between 360 and 600 liters, so it would be reasonable to get a horse out of a Force between 6 and 10. Not unreasonable when you consider that a Horse is strength and Body 8 on page 291 of the basic book.

However, it's an ally spirit, so you can jolly well just inhabit it into a horse, at any Force. And at any force a horse is a horse of course. So put your morse on a horse with a force and ride a course to the bourse and steal their gorse. It couldn't be worse.

-Frank

Frank... bad.

...and it could talk you, of course of course, engaging you in most meaningful discourse.

...ack! I think it's time to go back to the ranch now..

You should all be hung for crimes against the english language.