what is the best format to import icons from illustrator to indesign in?

so file place into Indesign? and this retains all icons to the same size across different files?

By @James3669094303ct

By default, they'll be placed at the size you've created them in Illustrator. 

But if you resize them after placing - then they won't resize automatically if you resize them in the Illustrator and update your AI file. 

You could create an ObjectStyle and set size - but again - it will depend on the original size in Illustrator and you'll have to update this ObjectStyle between documents.