How to get widget size in Unreal Engine

The problem is absurd. You can't just get a size of widget before it will tick for a first time. I've met this problem when was writing a custom grid component. The component holds a set of widgets and should align them in a grid with cols count evaluating dynamically, depending on a size that widget takes on a parent widget at the moment of creation. And thats the problem.

Finished Inventory Grid

I'm working on GUI system for Insomnia RPG. I've done alot of work but there is still much here.
Today I'm finished inventory grid widget that allows player to manage his inventory.

Players will be able to disassemble things to get resources that can be spent when crafting new stuff. We althogh have a barther system that allow to trade things with NPCs.

This widget is used on loot and trading screen as well as on character inventory screen.