None of the apps I own allow you to create a canvas based on size (inches) and dpi. They are all pixel x pixel based. You may end up having to do the math. On the other hand, since you're only working with one size you should only have to do the math once.
You could probably do the art in one app, then use Open In to copy it to the iDraw app; where hopefully you can adjust the dpi to properly reflect the size.
As for apps to use, I'm really digging the toolset in the SketchClub app. It's a surprisingly robust and versatile drawing app when you consider that it's main selling feature is the community. You can create a custom canvas at almost any size, but you lose layers as the size increases. Ata resolution of 4096 x 4096 on an iPad Air you only get 2 layers. You get 9 at the more common 2048 x 2048 canvas size.
Hold in mind I am not a professional artist; just a doodler who can sometimes fake stuff.
You can check out the range of art created using the app by visiting sketchclub.com
The other, more popular, artists tool for the iPad is SketchBook Pro. You'll find a lot of support for this app online, since is is available on several platforms. You can create custom canvas sizes here as well; though only about half the size as the SketchClub app.
There is one more possibility you may be interested in, since you mentioned your Illustrator days. InkPad is a vector based app that can export as PNG, PDF, and SVG; and you can specify canvas size in inches. The sample artwork is astounding (as it often is). How good is it? Well I don't know. I got it meaning to give it a try but haven't gotten around to doing so. It's gotten good reviews though.
I have no idea how it would fit into your workflow or whether that 6x6 inch export would also be 300dpi, so purchase at your own risk, or after more extensive research.
That's all I have to offer. Good luck on your quest for the art app you need.
