Are 19 3/8" mustache bars too small for a 22" Crosscut hybrid, with a 5'7" female riding it?
It'll have 9-speed Shimano bar-end shifters, which makes a slight difference...

They almost seem like children's bars, especially when placed next to a 21 3/8" pair - which I'd like to use but they are steel and quite heavy. (The bottom bars are the smaller alloy, the middle set are steel and a bit wider...the pic is distorted, and doesn't show this very well.)

I would say the width of bars goes by shoulder measurement/at least it does for me. I would go for the alloy. Is the id on the mustaches the same? If not, pick the one the barends fit

Try them. If she's comfortable and likes to ride the bike, it's all good. If she complains about something, change it.

Mustache bars are tricky. A lot of people don't like them. The trick is to find the perfect setup that puts the rider into exactly the position they like. A drop bar offers a variety of riding positions and among those the rider can usually find a position that's comfortable; the mustache bar is much more limited, and this makes it harder for the rider to adjust his/her position to suit.

I think with moustache bars it is OK to go slightly wider than shoulder width. But even so my 22" Crosscut has a 24" long top tube making the reach the more important number for a smaller rider. Which one of your bars comes the farthest back?

On a side note. I'm 6'2" and my 22" Crosscut has a standover of about 33" depending on the tires and my 5'9" girlfriend doesn't fit my bike at all.

Well, that settles that...the id on the alloy set is too small. Good thing the bars were only $2.00!

I'll either use the steel ones, or continue to hunt for an alloy set...

I can't remember what width I used but when I put mustache bars on a hybrid I found that I had to angle them up a lot more than I thought I would in order to get a comfortable hand position. I also found that I didn't like them for distance because there were fewer usable hand positions than drop bars.

However I like them for commuting; it's a more upright position but with more stability and flexibility than regular flat bars, plus they look 1000% cooler than flat bars.