When you choose File > Place and place an AI file, it will be placed by default at 100%. So if you needed the AI file to exactly fill an INDD frame, you would need to know the size. If you don't need to be precise, that wouldn't be necessary.
I have no existing Illustrator files for the project. I'm tring to use InDesign to create a rapid protoype. I may be working backward from what is expected.
If I could generate blanks that are the correct size that would be an easy way to get started on them, but copying the sizes to the new document dialog box in Illustrator is not a big deal either.
I figured that if i could create an Illustrator blank from InDesign that would get me started in illustrator quickly and also place the new file in the frame.
Depending on how you choose to crop, there's no reason you can't place any size Illustrator file and fit content to frame.