I don't use the latest version of igrid and therefore apologize if this problem has already been resolved in the most recent versions.
Using the iGrid.NET filter, I have a resizing problem. The first time I open it, the window appears as in image 1, closing and reopening the window several times (not many, 10 are more than enough) the window becomes smaller until it becomes as in image 2.

How can I solve it?
Thanks to who will answer me

Yes, this issue was fixed in v6.0 build 0012 released on May 18th, 2018.

Here is the pertinent excerpt from the What's New:

In apps compiled for .NET Framework 4.5 or higher, the filter box dialog of AutoFilterManager decreased in height and width by 8 pixels with every opening. The issue was related to the implementation of SystemInformation.FrameBorderSize in .NET that returns incorrect values in .NET 4.5 and higher versions.

To find out more about this issue in .NET 4.5+, read the following StackOverflow discussion:

The SystemInformation.FrameBorderSize return wrong size