sizing a riding helmet for a 8 year old ???

I would take her to a tack store that sells helmets and have her try on different sizes. A properly fitted helmet should be snug, but not uncomfortably so. It shouldn't slip down over the eyes or sit too far back on the head. A good way to test proper fit is to have her put on the helmet, buckle it, and shake her head up and down and back and forth. If the helmet moves around, it is too big. If she says it gives her a headache, it is probably too small. Just make sure the helmet is properly certified to be safe for riding. I forget the name of the certification, but the tack store owner/worker should be able to tell you. That will also save you money in the future if she decides to get into showing, as most shows nowadays require use of a certified helmet. Just as long as she doesn't grow out of it before then! I remember when helmets were $80 though and I always thought they were the most uncomfortable, heavy things. Now adays you can get a nice, lightweight, comfortable, well-made helmet for $30 or less. I ride in an endurance helmet because it has good ventilation and is lightweight and doesn't look like a mushroom on top of my head. Her preferred style of riding may determine the type of helmet you need to get her.

It is so totally super that you want to get her a helmet.

Sizing is important but so is shape.  Some helmets have a kind of longer oval shape, others are rounder - the shape has to fit the person's head as well as the size.  The helmet actually should have its bottom edge, quite close to the child's ears - like in the first link.  If you see a lot of the child's head above her ears when the helmet is on, it's not going to offer much protection.

One helmet certification is ASTM, but some folks suggest the standard, even, is pretty minimal.  I'd suggest a helmet like a Charles Owens (there are less costly helmets than Owens, with the same design, harness and materials) that is durable, has a well designed, adjustable harness, and is very comfortable.  They also have padding at the back that is designed to protect the lower part of the back of the head.

The 'very well ventilated' helmets with big holes, the cheaper helmets, usually aren't certified, and often provide much less protection.  Some protection is better than none, but the trend over the last 10 yrs or so, is to have more of the sides and back of the head protected, as many injuries do occur at those points.

With the harness and chin strap adjusted properly and snug, the helmet should sit level on her head, not tipped back,

This one is sitting level (though the design of the helmet slopes down a little at the back, a good feature)  -

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This is what is meant by 'not sitting level' -

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The helmet in the first pic might look a little big, but I think it actually just has thick padding inside, which is good.

and not move when she shakes her head (mentioned already). 

If buying used, see if you can stick to a newer looking helmet that doesn't look like it's taken a fall.  Ideally, each little youthful noggin would be in a new helmet.

My daughter uses an Aegis Youth. It has a warranty with it too. It's also adjustable, a good thing for my rapidly growing daughter!

The part I highlighted is certainly not true in all cases. The certifications on my endurance helmet are the same as my old "tank" helmet. (The tank helmet was considered top of the line 15 years ago. That thing was expensive and heavy but I used it for years showing in 4-H) The endurance helmet also covers more of my head, despite the ridges for ventilation. These helmets are a popular choice around here for western showmen, as more and more shows are requiring the use of helmets instead of cowboy hats.

I would perhaps ask around the barn where she rides (if she rides at a barn) and see what other people are using. The employee at the tack store should also be able to help you. My helmets have certainly saved my head on several occasions so it is important to have one that fits her needs.

I know an eight year old would like an actually helmet under the tree, but half the fun is shopping in a tack store. Cut out a picture of a helmet and wrap it up with some other small horsey gift. Then after Christmas go helmet shopping and get one that she really likes and that fits correctly. There is nothing worse than a helmet that does fit well, some of us have oval shaped heads and some more rounded, it can make fitting a helmet more tricky. Lots of fun colors are available for the younger horsewoman now, she will have a great time shopping.

Troxel makes a good helmet, or at least they used too. And theres plenty of little helmet caps to make it pretty and fun looking.

I just bought a Troxel helmet for my 6 year old and it works just fine.

I wouldn't buy a used helmet online if I were you. A bad fall or even a good whack of a tree branch on a trail ride can make the protection power of a used helmet iffy.

For me $55 was NOT too much to spend on my daughter's safety. You should be able to find a youth helmet around that price. Yes, velvet hunt style helmets are usually closer to $100, but again, my daughter's head is worth more to me than that.