Inputs should be easy to hit

Using smaller-sized inputs might sound like a brilliant idea to save space, especially on mobile. However, small inputs require more effort as it's harder to hit the type area. Plus, forms with multiple tiny inputs are far more difficult to scan. Setting a height between 32px and 40px makes an input visible and clickable, but not very big at the same time.

If you're afraid the form may become too lengthy and require users to scroll, break it into logical steps. The only caution is to keep the number of steps up to 5-6. Otherwise, the form may look too overwhelming.