I looked and found over a handful of grand duos models so it's a safe bet that from model to model and from carrier to carrier the "exact steps" can vary. This means that you have to get back to the carrier that sold it to get exact steps.
As to the message text size, http://android.stackexchange.com/questions/42403/how-can-i-decrease-the-font-size-of-incoming-messages-on-the-samsung-galaxy-note notes a text size change with the volume button.
Bob
I have a galaxy grand duos phone and need to set GMAIL as my default email. Also the text size of the message increased which i am unable to change. Please help.

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