On the article page of the respective skates, you will find a size chart from the manufacturer in the article pictures (usually the 2nd picture). To measure your foot length correctly, stand with your heel against a wall and measure from the wall to your longest toe. Add 5 - 9mm to this measurement and compare it to the Mondo measurement in the size chart. This minimises the risk of your new skate being too big or too small before you buy it.
In the example below, the skater has a measured foot length of 275mm. Adding the 5mm gives him 280mm. With the selected skate, this would be an EU 44.