What would be a fair annual increase in salary based on merit?

Is 5% considered the minimum acceptable increment, or should one strive for a higher percentage?

Well depends on industry, generally speaking a 3 - 5% increase is pretty normal. Of course, you could renegotiate your pacakage with your manager, however do not ask if you have nothing to show. Build up your portfolio and then review your package with your manager showcasing the different work you have help the company with. That would help a lot. All the best!