Exactly a geggeg said.
You know that the screw size is M3, the x30 / x35 is the length in millimeters, so just measure up how thick your fans are plus thickness of whatever else the screw needs to pass through, allow a maximum of 5mm extra to go into the rad, and the resulting length is what you need.
If it's just to go through a case side with a fan in between the case and the radiator, then I'm guessing the fans you're using are "slim" fans, probably 15mm? If so then you want M3x20 screws.
Another option in this standard layout would be to use 5mm M3 screws (M3x5) to attach the fan to the rad, then use standard fan case screws (the thick but stumpy self tapping ones) to attach the fans to the case. But of course that is only an option if your fans have "open" corners, for closed corners you can only use screws that go all the way through to the rad.
In that case you need M3x25 screws, but keep an eye on them as you're screwing them in and if the ends look like they might damage the rad fins / tubes then file them down a mm or 2.
You can also cut down the ones you have if you want, just put them in a vice or pliers and use a hacksaw, making sure not to damage the threads too much at the cut and after careful measurement for length.Edited by DaUn3rD0g - 5/18/15 at 4:55pm