Drawn with a mouse, 3 filters applied, and colorization. >_>
But still, they're NEON and they're kinda cool.

They can be any size you'd like. They do give off a faint heat, and they glow (and therefore look best on a black / dark background). The key thing to remember is that they're made of neon encased in glass - they can break and are extremely unlikely to be ridden unless it's by a creature that won't damage the glass. They're reasonably durable, but a solid strike on them will shatter that portion. Wherever there is a gap, that's actually a separate piece of neon. They aren't technically "solid", they're 3d only in that the glass is rounded... Otherwise you can see right through them. They have no organs, though they could be pursuaded to "breathe fire" (in the form of a blinking flame-shaped sign...).
They can be male or female or whatever, though I think they'd require either complicated magic, or raw science to reproduce or be fused with another creature. Traits they may pass on would be glass shell, glowing, and 'outline' areas. (i imagine that a solid + neon dragon offspring might have a normal body but all its accessories like claws, spikes, maybe even wing sails would be detached neon.)
I have 14 available!
Choose your color, I'll try and update as people choose.
Orange -- Snapped up by Maerwynn
Gold -- snagged by Dray
Yellow
Lt Green
Dk Green
Aqua -- I'm gonna take this one for myself.

Sky Blue -- snagged by Starburst
Cyan -- Taken by Starfyre
Blue -- snagged by Kayla
Indigo -- snagged by Uny.
Purple -- dibbed by Cy!
Heliotrope -- taken by Kendarath
Pink
Red -- grabbed by Birdie