I have a Bailey Mid Arm and it fits on my Grace Frame.

I have a Juki 98Q with a 9 inch throat that I consider a mid-arm. So I would say that the definitions vary with the folks who have opinions. I have mine on a Grace GMQ frame.

My Babylock Ellegante at a 9" throat is considered a mid-arm machine. I was told by the dealer the mid-arms start at 9".

They have some of the length between needle and pillar, but they certainly lack the height between machine bed and underside of machine compared to an HQ16 for instance. The height of my HQ is approx double that of the 820/830.

Several companies make midarm machines...
Bailey Home Quilter- can be used either sit down or on a frame and they come in 13", 15" and 17"
WoW sells a stretched domestic machine
Voyager is a 17" machine and is VERY heavy we're talking 75+ pounds but it's solid.
Handiquilter sells the 16" machine
Nolting has a few midarms as well.
Janome, Babylock, Viking etc have them as well. Pretty much just about every company has them - minus Singer but I don't think highly of the new Singers so there is no love lost there imho.

It all depends on how much money you want to spend. If you are looking to get more quilting space I would bypass the 9" machine if it were me and get at least a 15" or 16" machine

Great thread, when I upgrade my machine this is what I am going for. I cannot justify a longarm right now so I think a midarm is the best option. :)

One thing to keep in mind, only strait stitch machines set up on a table can have a stitch regulator added.
I have a Sapphire 830 with a 10 in throat and needle down but you still have to shove a quilt thru it. I paid $1100 for it new a few yrs ago. It helped but its not a long arm.
I cant afford a long arm machine but can do a strait stitch on a frame so I get mine on Mon. These machines are reasonably priced, The Brother runs around $600. The stitch regulator runs $500. With a table it will run around $2000 for the set up.
Of course you need the room for it. Mine will be taking up the dining area of my living room.

Bernina has a bit of technology they call the BSR - Bernina Stitch Regulator. Their machines can be placed on a frame, or used as a sit-down stitch regulated free motion machine. That being said, they're *hideously* expensive - my 440 in Canada costs nearly $4,000 new. The Mid-Arm 820 and 830 also come with a BSR (I think), and they can be placed on a frame as well (In fact, Bernina does sell a frame for these machines, but I've heard any frame will work.)

The BSR is tricky to use, and can sometimes not produce the world's most consistent results. I get a lot of skipped stitches, and if the fabric is very plain or very shiny it doesn't work very well. (I'm fairly certain the technology is similar to that which is found inside an optical mouse.) It's wonderful, though, if you're working on small projects (like coasters, quilted pillows, or bags) that are impractical to load onto a frame.