256GB is largest size, cost about 400 beanies though.
I've used my 2013 MBA with and without the PNY 128GB, barely notice a difference. There isn't really major power draw when it just sit there. When I was formatting and encrypting or decrypting it and using max IO for hours on end, then yeah, noticed the battery dropped a little faster.
Major thing you have to watch out for is there was a bug where having the SD card mounted would not allowed computer to go into deep sleep. Battery dropped a few % overnight when that occurred, otherwise if it work correctly there's no noticeable power drop. I believe that bug got worked out, I've disabled my workaround for it and seem to be fine now.
One thing I would caution on is not to put a lot of small files, and only things that are backed up on the SD card. I ran into several hard IO error that corrupted the file system on the SD card and ended up losing entire folder structure. My original use for this was to put about 1.5M small graphics in a library on there where I could browse around. It did not liked copying hundred of thousand of small 1500 bytes files... Long term I/O of small files triggered the hard failure as far as I was able to reproduce the condition. Happened with 2 PNY cards (original, and a replacement).
I still use it, and has been great for about 6 months, but now I only put things with much larger file sizes and fewer files, and things with sync backups like Dropbox, or a copy of photos, or large DMG which I have copies of on there.