Unfortunately, without knowing the make and model (which is usually found on the back of the unit), there is no way to tell, there are a few different screw sizes commonly used in laptops and some (like the HP machine I'm currently working on) use different sizes just on the back. A picture of the back of the machine (where the screw is missing) may be of help but even then, unless we can identify the laptop, it's likely we won't be able to say for sure what size the screw is.
A size 1 Phillips will get you through most laptops but you may come across a few screws which call for a size 0 Phillips (again, manufacturer/model dependent)