So set the Full size in MathType's Size menu to be whatever size you're using in PowerPoint – probably somewhere between 24 and 36pt. You may not be able to tell they're different sizes, but your viewers will be able to. If you're in the habit of inserting small equations into PowerPoint, then dragging to make them larger, no two equations in your document will ever be the same size.
We assume you don't want them to be 'good enough' you want them to be as good as they can be. One, the whole reason you're using MathType is so your equations will look their best.
NEVER leave MathType's font size for PowerPoint slides set the same as for Word documents, then insert them into PowerPoint and click & drag to resize them.
