I have a tab control and a user control with a button. When I add the user control to the tab with this code:
ProductImporters uc = new ProductImporters();
uc.Dock = DockStyle.Fill;
Instead of DockStyle.Fill use DockStyle.None. Use Anchoring to fix the position of the button. For example, anchor to right and bottom to fix it on the bottom right position. Set Autosize property of the button to false. This will avoid resize.