Is it possible to reduce the height of some or all fields ?

It is possible to reduce the height of the form fields by injecting custom CSS code. Here's the code:

.form-textbox { 
height: 10px !important
} 

.form-textarea { 
height: 10px !important
} 

Note: You can change the 10px to any number value you prefer. Here's a guide: How-to-Inject-Custom-CSS-Codes

Hope that helps. Let us know if you need any further. Thank you.