14500 with Nano Biscotte?

So I'm planning on making an incredibly small lightsaber for my son who's a toddler, so to satisfy both his desire for a saber that isn't comparatively humongous and my desire for various character hilts, I'm going to make a Yoda saber, which, if all goes well, will end up at about 5.75". I know, I'm a lunatic.
Anyway, I was planning on using a Nano Biscotte (Biscotti?), and while 18650 batteries are generally recommended (and I already have some on-hand in preparation), for the sake of conserving space, what would be the downfall of using a 14500 in this single instance? And if that does end up being the route I take, would it be best to use the AA battery holder, and a dummy cell in one side so as to not overpower the NB?
Thank you for your assistance!

The downside to the 14500 over the 18650 is runtime.

Are you using MHS v1 or v2. I'm thinking that the 2 AA battery holder wouldn't fit inside v2.

As rlobrecht mentioned runtime is the biggest difference. Also if you plan on dueling with the saber I would stay away from battery holders and just wire with a recharge port -- see Madcow's videos if you have questions/concerns he explains and demonstrates the entire process :). The batteries tend to lose connection in the holder when dueling causing your sound board to reboot.

It's possible that you may not be able to suck enough juice out of a 14500 to fully power all of the components of your saber. The maximum continuous discharge rates of 14500 cells can be a little low for an LED saber.

Good point. You should be ok with an NB and a single die LED, or a basic stunt and a single die LED.

I think for a saber that small, I'd go with a Pico and 18500 setup. Then wire for recharge. You lose the some flexibility of the Nano, but you can fit it all in a super small space, also pretty easy RGB if he decides he wants a different color blade later. Hopefully TCSS will start to sell chassis discs for the Pico. or you could just tape it to the battery.

Lots of input, and it is all aprreciated!
First of, I'll get more specific with the saber—it will be a single LED, no FoC (it just doesn't appeal to me), and, as rlobrecht correctly guessed, it will be in MHSV2. As for right now, my son (who is just old enough to be considered a toddler, at a year-and-a-half) will be content with anything "dey-buh" he can get his hands on, so other colors are a far-off concern, that will probably be met with another saber down the line as he grows. This in mind, battery life isn't a grand concern, as long as it's not something ridiculous like five minutes. And being as young as he is, he won't be doing any serious dueling for a long time [as Sir Alec Guiness]... A long time... [Back as me] And I highly doubt I or any of my compatriots will do anything more than a playful swish of a saber that will be less than two feet long.
Right now my primary concern is the amount of room vertically, which led me to think toward the 14500, and again, rlobrecht was right—there's not enough room in MHSV2 for a 2 AA battery holder. For the sake of weighing out my options, what if I went out of TCSS and used a single AA holder? (I found one on eBay for $.99, so it's not a huge bank-breaker.)
And I may just use the Pico, and I honestly go back and forth between it and the Nano all the time (plenty of thinking time as I await the restock of MHSV2 parts). Now that I look at it again, the situating of the speaker connections may be more suitable to prevent wires laying on top of wires and conserve space. Regardless of which board I use, it's going to be taped to whichever battery to conserve space.
This game of cram-fu is real, you guys.

I did a rgb NBv4 saber with illuminated AV on a AA sled with a dummy cell. Worked just fine. :O

Awesome! Was it all right in terms of brightness, volume, and battery life?

Battery life was shortish, depending on the color. Bright and loud enough. It might not have been optimal but it worked for the recipient's purpose. (cosplay at cons, hence the easy-to-swap battery setup.)