A 3.5" drive would make a laptop pretty thick. It would
end up as more of a desktop replacement style laptop if
that was the case (a laptop you don't really
want to carry around with you, that stays at your desk).
So yes, some kind of 2.5" drive is likely. 2.5" drives come
in a few different heights, and you want something that
will fit in the drive tray.
http://en.wikipedia.org/wiki/Hard_disk_drive
"Drives 9.5 mm high became an unofficial standard for all
except the largest-capacity laptop drives (usually having
two platters inside); 12.5 mm-high drives, typically with
three platters, are used for maximum capacity, but will
not fit most laptop computers. Enterprise-class drives
can have a height up to 15 mm. Seagate released a 7mm
drive aimed at entry level laptops and high end netbooks
in December 2009."
Be wary of any laptops that have special requirements of
the drive. Like, drives that need to be tattooed. Drives
that have the "free fall sensor". That sort of thing.
A business laptop is more likely to have features like
that, than a cheesy consumer laptop.
Interface types on the hard drives, include IDE (44 pin, 2mm spacing),
and SATA (connector is the same as a 3.5" SATA drive). So that's
another variable to check.
Some storage devices in laptops, include an "adapter". A plug
that fits on the drive. As near as I can determine, these
may exist to make the mechanical connection between
drive and laptop, more flexible, so if the machine
receives a shock, the connector won't snap off.
So there are just a few variables...