Europe
From http://www.egginfo.co.uk/page/eggsizes:
|-----------------------|
| Size | Weight |
|-------------|---------|
| Very large | >= 73g |
| Large | 63-73g |
| Medium | 53-63g |
| Small | <= 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 |
|-----------------------------------|