The website does a calculation from a person's height. Height is entered under My Account under the Health info. If you're height is correct, then your estimated stride lengths are found on the Body Tracker page in the Walkingspree website on the links of the left hand side. This number is what you enter in your pedometer, not your height.
If you want to determine a more accurate stride length, then you need to follow the instructions in the Omron Pedometer Manual on your program's login page at http://support.walkingspree.com/attachments/token/joujg4ivdw2nf0r/?name=HJ-324U+Manual.pdf. It will walk you through measuring a set distance and counting the actual steps you take.
Some general information about stride length. Your stride will vary depending on your speed, ie. running strides are often longer than walking strides. Most people do not update their stride in the pedometer when changing activities so it means your distance may vary a bit.
The stride length is only used for distance and does not impact your step count. The website displays distance but is not ever used for anything in an official capacity because of the variation in stride lengths among participants. so distance is primarily for personal use only.