Adding space is possible with padding and margin in css. More information @ here. You can apply your css rules by giving your form a custom css class. Add this css class in your theming stylesheet to apply the css rules.
An alternative is to add the css style in the modeler to a specific table cell or label. Just select a cell or label in your form designer and find the style property. Add your own css style, something like this:
A disavantage of this method is that you need to redeploy to see the results. The preferable way is to define a css class in your form and use this custom css class in your theming. You can do this by modifying the stylesheet in your theming. Reading the theming documentation could help you understanding how theming in mendix works: https://world.mendix.com/pages/releaseview.action?pageId=11437788
Use the Format String Widget from the Appstore
if you set it to div mode and put <br/> into it it will create nice whitespace on the form.
The missus swears by it to create eye-pleasing forms !