I found it myself. It is specified in the S90 style guide available here at forum nokia.
Copied from the style guide:
The standard sizes, in pixels, for the various icon types that are used in the Series 90 user interface are as follows:
• Application icons in Desk:
Medium and maximum zoom level: 64 width x 50 height.
Minimum zoom level: 25 width x 20 height.
• Notification icons:
Small version in infoprint: 25 x 20.
Larger version in other notifications: 50 x 50.
• Toolbar icons:
32 width x 32 height in applications. This is the size of the Toolbar icon placeholder, but icons do not need to use the whole of the available area. An icon can be surrounded by a border of empty space in the transparent mask color and thereby appear smaller than it is. Icons in the Toolbar are often 25 width x 20 height. Margins between Toolbar icons are dynamic, but the minimum is 10 pixels.
• Icons in lists: 25 width x 20 height.
• Icons in controls: 25 width x 25 height. Icons can also be used purely as visual support for the text, for example in a button, these are 25 width x 20 height.