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.