I have seen beautiful large projects (HAEDs and Golden Kites) stitched on 18 ct Aida fabrics. Whatever works for you is right!
That's my hang up -- I think the HAEDs look so much better with small stitches! I think 18 ct would be ok... although now that I've been using mostly evenweave type fabrics, when I do go back to aida, it feels really stiff. Maybe a 36ct linen, stitched over 2? Never tried that before. My "ornament" sized HAED is going to be a little bigger than I anticipated! Maybe I'll make a small pillow out of it with a loop on the top, and hang it on a doorknob. I just can't face that 25 ct! Maybe on a weekend morning, but I do most of my stitching in the evenings, after working on a computer all day - my eyes are toast by 7pm!
Anyway, I stopped with the HAED ornament & went back to starting another mason jar lid. I'm doing the MS Walk in April and decided to sell all the jars I've made so far as a fundraiser, so I'm frantically stitching a handful of Easter designs over the next couple of weeks so it's not all Christmas jars. I'll fill them with Hershey's kisses, too. Hopefully people at work will buy them!