Your problem is you have:

foreach post where A is true, that has a child where B is true

Which is actually:

foreach post where A is true
    that has a child where B is true

So this is not something you should be doing in 1 query. Instead you would need to do n queries where n= parentposts+1. 1 query to find the parents, and then a query for each parent to find the children.

Clearly this is not going to be fast.

The Root Of The Problem

Your data structure is inappropriate. You’ve attempted to do is reinvent taxonomies using post meta.

Instead, create a custom taxonomy called ‘Location’, and create a Stockholm term. Then do a query for all posts that have the meta for size, and add 'Location' => 'Stockholm'.

Benefits:

• Querying is dramatically simplified
• You’re given a working, extensively tested admin interface for free
• Its fast
• You get free archives, e.g. example.com/location/stockholm/
• Tight integration into the template heirarchy with new templates: taxonomy.php, taxonomy-location.php and even taxonomy-location-stockholm.php

Disadvantages:

• None
• Nothing
• There aren’t any

As a sidenote, if you run into a situation where you need to execute an sql query, you’ve done something wrong.