Use the pad options (padx, pady, ipadx, ipady) for the grid command to add padding around the text widget. For example:
self.text.grid(row=0, column=1, padx=20, pady=20)
When you say the tag command doesn't work, how do you define "doesn't work"? Are you getting an error? Does the font look big but not bold, bold but not big, ...? The command looks fine to me, and when I run it it works fine.
Your example doesn't show that you're actually applying that tag to a range of text. Are you? If so, how? If you do the following, what happens?
self.text.insert(1.0, 'is this bold?', 'big')