No Apple are evil and they released computers in October 2010 that are utterly obsolete in Summer 2011.......
😉
But in all seriousness. Yes your MBA should be fine for Lion, no one knows about install size, but you can always shave off a few gigabytes of installation size by custom installing, and getting rid of printer drivers and foreign languages.
no, once a lion sinks it's jaws into a MBA it's be toast, have you see the fangs on a lion? 😛
In all seriousness, I don't think the OS upgrade will be a huge jump in size, I'd be more worried about running out of space for other files etc if you had the 64GB MBA.
The Snow Leopard installation is only 12-20 GB in size, depending on apps installed, so there is considerable room available on a 64 GB disk (unless you have huge music or photo libraries). I doubt Lion will be significantly different in installed size, so you will be fine.