Works on kithnyc.com. Not sure about any other Shopify site.

copbookmarklet.js

(function() {
  var size = 'YOUR SHOE SIZE HERE';
  var subdomain = 'shop.',
    path = '/cart/%ID%:%QUANTITY%';

  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState == 4 && xhr.status == 200) {
      var parsed = JSON.parse(xhr.responseText);

      var found = parsed.product.variants.filter(function(o) {
        return o.option1 == size;
      });

      found = found ? found[0] : null;
      if (!found) {
        alert('Your size is not available. \
          Don\'t forget to replace it in the beginning of the script.');
        return false;
      }

      var replacements = {};
      replacements['%ID%'] = found.id;
      replacements['%QUANTITY%'] = 1;

      var url = subdomain + document.location.hostname + path;
      url = url.replace(/%\w+%/g, function(match) {
        return replacements[match] || match;
      });

      document.location = document.location.protocol + '//' + url;
    }
  }

  xhr.open('GET', document.location.href + '.json');
  xhr.send();
})();