When you create your usercontrol
For the textbox
1)set location of the text box to 0,0
2)set the margins of the text box to 0
for the usercontrol,
1)set autosize to true
2)autosizemode = grow and shrink
3)margins to 0
then the size of the textbox is equal to the size of usercontrol
you build your project
when you now place your user control on the form, you should see the size of the textbox is the same