Thank you for quick reply! I just saw that you were using that 'imgui_h_button' in your example, my bad. I need to be more attentive. I was previously using imgui_button(text), which doesn't allow to set the size, but only created a button, which will change it's size automatically depending on the text length.
Small tip: just found out how to remove close button from the window, without using NoTitleBar flag seton. In 'imgui_begin(char *name, void *p_close, long flags);' simply pass NULL for p_close.