Update: As of v3.6.7 of in5, the Font Size can be set in the Buttons and Forms panel within InDesign.
For now, you can apply the following code as a .css file in the Resources section of the in5 dialog:
input { font-size: inherit; }
It will override the browsers default font-size for the input and make it a more normal size.
This code will automatically by applied in the next in5 update.
I'm also working on supporting the Font Size setting that has been added to the Buttons & Forms Panel in recent updates.
Alternatively, you can give the text input a specific font size (or other properties), e.g.,
input { font-size: 16px; }