I agree - my 10Mp camera does a good job up to 11"x14" (roughly A3), but then struggles (I have a large printer and
could print to 16"x over 40"). Bigger prints require bigger images - and resizing in Photoshop or equivalent can only do so much.... as you increase physical dimensions of an image you reduce the resolution - and if it drops below 200dpi you've had it.
If you're going to be regularly printing bigger in the future there are two options: A bigger Sensor on the camera or investment in software like
Genuine Fractals (now called 'Perfect Resize') but it's not a cheap plug-in.
I've used a third method on occasion, but it's something that needs planning.... making one large picture from 2 or 3 shots (each taken in 'portrait' orientation). Join them by editing them on layers or just go for a panorama. That 10Mp picture can actually be closer to 30Mp once three of them are joined up.