Battlesuit Alpha was not written with any particular miniature line in mind. You can use whatever miniature mechs you have. There are many popular miniature lines out there to choose from, like Battletech, Cav, Heavy Gear or 3d printed options. Some are very cheap on discount sites. Most will be 6-10mm scale.
The scale of your terrain will determine how big your mech looks on the table.
Using 15mm terrain with smaller scale mechs will make them smaller than the buildings, darting between cover. Using smaller scale terrain with larger scale mechs will make them look like they are towering over the surrounding buildings.
Decide what looks right for you.