Running LR in Macbook Air

Here's a few things to consider.

AS for the 11" model, you might find the screen resolution tight for LR at 1366x768.  The 13" provides 1440x900.  Every pixel helps for viewing images and to reduce the scrolling on sidebars.  Compare for sure before you buy.

I've had my 13" MBA since 2011 for travel.  While it maxed out at the time at 4GB - it's workable but yes, 8GB would be better.  When I updated my spouse to a MBA last year we went with 8GB/512 and it runs LR very nicely.  It has an SDHC slot and coupled with higher speed cards, can read them at roughly 70MB/sec.  Important for higher resolution cameras.

Now as to storage - there are several solutions.  But start by considering a 256GB model and using an external drive you may already have for image storage and keeping JUST the LR Smart Previews on the system drive.  256GB will provide you plenty of space configured this way if you haven't loaded up a lot of other data.

If you really like this route and need to get a new external drive, consider an SSD (no moving parts and better reliability for travelling).  Either USB3 or Thunderbolt - although the Thunderbolt option will be pricier and, in the field likely not much of a speed advantage although some.

An alternative is to augment the system drive space with one of the SD slot drives available from TrendNet and others that mount a micro SD card flush with the case and provides you with up to another 128GB of storage ... and you could have two or more for additional storage.  However, those are not that easy to back up to one another as you only have one slot, and once you've taken up the slot it's not available for your camera card so you'd need to tote around a USB3 card reader.  So, while this is an alternative, there are perhaps more cons than pros to it in practical terms.

Finally, cost out the solution then go back and review what the storage upgrade from 256 to 512 is and see which is more cost effective or convenient.

In the end, you still need some form of backup.  Don't over-write your cards, maintain a copy on the system drive (or primary external drive), and then one other copy on something like USB thumb drives, flush-mount SD drives, etc. That's 3!

When being used as a travel machine being lightweight is pretty much the trump concern (esp if on small planes); having no mechanical hard drives is important for reliability; finally as it's a travel machine backup is really important owing to the number of ways you can lose your images such as theft, loss, mech failure, water/sand.

Leave the culling till you get home (e.g. don't erase); but have fun using the 13" screen for LR.

Lastly, I am looking to upgrade mine this year - but I am going to wait just a little while longer to see what the implications of LR's next release will be and what Apple may have as a replacement for the existing MacAir line.  Two big unknowns with lots of promise.

As for catalogs - you have a couple of options.  You can travel with a bare bones catalog just for the trip and then import all of that back into your home-based catalog when you get home using the Import from Catalog file option. I don't recommend maintaining a lot of independent catalogs because you lose so many of LR's benefits when doing that - unless you have a really specific reason for maintaining separate catalogs (e.g. your a wedding photographer and you keep each client in a separate one for obvious reasons).

‌I do use an 11" MacbookAir with LR when traveling.

Correct, it does not have an SD-slot, so I use a card reader.

The biggest disadvantage is my fairly small internal SSD (256GB) for me, so I use an external SSD for LR catalog and previews + smart previews, plugged in via Thunderbolt connection. This is nicely quick.

You might use another external HDD for actual images, in case you shoot many GB of RAW images and might get into disk space problems with the internal SSD. USB3.0 is also nicely responsive for that. I reckon with 120GB of new images per vacation trip, shot by two photographers.

And yes, 8 GB of RAM are essential.

11" screen is quite ok, if I use the develop module in mono mode, i.e. only one section open at a time.

‌Personally, I don't use Macs so I don't have direct experience, but the most important thing for running LR on an up to date computer is to have enough RAM; this is more important than processor speed. You need at least 8 GB if you want it to generate and display pictures.