Converting between Egg sizes
Europe
From http://www.egginfo.co.uk/page/eggsizes:

|-----------------------|
| Size        | Weight  |
|-------------|---------|
| Very large  | >= 73g  |
| Large       | 63-73g  |
| Medium      | 53-63g  |
| Small       | &lt;= 53g  |
|-----------------------|

USA
From http://www.georgiaeggs.org/pages/sizeequivalents.html:
Size equivalents
US recipes using eggs, but not specifying size, are referring to large eggs. Here is a substitution table:

|------------------------------------------|
| Large | Jumbo | X-Large | Medium | Small |
|-------|-------|---------|--------|-------|
| 1     | 1     | 1       | 1      | 1     |
| 2     | 2     | 2       | 2      | 2     |
| 3     | 3     | 3       | 3      | 4     |
| 4     | 3     | 4       | 5      | 5     |
| 5     | 4     | 4       | 6      | 7     |
| 6     | 5     | 5       | 7      | 8     |
|------------------------------------------|

Here is a cup equivalency table as well:

|-----------------------------------|
|            TO MAKE 1 CUP          |
|-----------------------------------|
| Egg Size | Whole | Whites | Yolks |
|----------|-------|--------|-------|
| Jumbo    | 4     | 5      | 11    |
| X-Large  | 4     | 6      | 12    |
| Large    | 5     | 7      | 14    |
| Medium   | 5     | 8      | 16    |
| Small    | 6     | 9      | 18    |
|-----------------------------------|