How to set font size for text of dialog buttons

Instead of returning builder.create(), try this.-

final AlertDialog alert = builder.create();
alert.setOnShowListener(new DialogInterface.OnShowListener() {
    @Override
    public void onShow(DialogInterface dialog) {
        Button btnPositive = alert.getButton(Dialog.BUTTON_POSITIVE);
        btnPositive.setTextSize(TEXT_SIZE);

        Button btnNegative = alert.getButton(Dialog.BUTTON_NEGATIVE);
        btnNegative.setTextSize(TEXT_SIZE);
    }
});

return alert;

You can try this:

AlertDialog.Builder builder = new AlertDialog.Builder(CompQuestionsActivity.this);
builder.setMessage("Message");
builder.setPositiveButton("Yes", dialogClickListener);
builder.setNegativeButton("No", dialogClickListener);
AlertDialog alertDialog = builder.create();
alertDialog.show();

//For positive button:
Button button1 = alertDialog.findViewById(android.R.id.button1);
button1.setTextSize(25);
//For negative button:
Button button2 = alertDialog.findViewById(android.R.id.button2);
button2.setTextSize(25);

Took me a while, to integrate Asok's answer, since I used anonymous inner classes for buttons, so I needed to get a handle on the button references. This works. Make sure it goes after the messageDialog.show() line:

messageDialog.show();
messageDialog.getButton(AlertDialog.BUTTON_POSITIVE).setTextSize(TypedValue.COMPLEX_UNIT_SP, 25.0f);
messageDialog.getButton(AlertDialog.BUTTON_NEUTRAL).setTextSize(TypedValue.COMPLEX_UNIT_SP, 25.0f);

Note: It's recommended to use sp as a unit for text size. Unlike px, it is device density independent.

Since you are already using an xml file for the dialog why not just include the two buttons in the layout and set the onClick handlers in the dialog creation, something like this should work. I am using something similar.

Here is a quick example:

AlertDialog.Builder builder = new AlertDialog.Builder(getActivity()); 

View view = getActivity().getLayoutInflater().inflate(R.layout.dialog_login_confirmation, null);

TextView message = (TextView)view.findViewById(R.id.txtLoginConfirmationMessage);
message.setText("Are you " + empName + "?");

Button positiveBtn = (Button) view.findViewById(R.id.dialogButtonPositive);
    // Set size of button in relation to screen size
    positiveBtn.setTextSize(TypedValue.COMPLEX_UNIT_PX, (float) 25);
    positiveBtn.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            mListener.onEmpConfirmPositiveClick(LoginConfirmationDialog.this);
        }
    });

Button negativeBtn = (Button) view.findViewById(R.id.dialogButtonNeg);
// Set size of button in relation to screen size
negativeBtn.setTextSize(TypedValue.COMPLEX_UNIT_PX, (float) 25);
negativeBtn.setOnClickListener(new OnClickListener() {
    @Override
    public void onClick(View v) {
        mListener.onEmpConfirmNegativeClick(LoginConfirmationDialog.this);
    }
});

builder.setView(view);
return builder.create();

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I am also quite fond of using the following for setting text sizes, this allows for various screen sizes to get a different size of text (You can play with the float value to suit your needs):

.setTextSize(TypedValue.COMPLEX_UNIT_PX, (float) 25);