Hello!

I would like to hide some of form items depending on the data, user enter to the form. I set False or True to the Visible property for form items and they change visiblity. But the form changes its size too! That looks annoying. What can I do to make form have a static size?

May be Im wrong, but I noticed the following
try set property visible for all form items to true, and change property visible in OnCreateAtServer event handler. So then form size not change. But if i set property visible to false in properties pane in designer then after item's visible changed to true, form size changes

Is it a bug or a feature?

It is a feature. Form size grows than you need it to, but when you want to hide something form size stays the same and just rearranging items. It also depends a little of platform version. I have noticed a little different behavior checking 8.3.

You can use pages to group your form items and hide entire the page to keep form size a bit more stable.