There's a little known formula used by pros for increasing the size of a print using the minimum of 'interpolation' (adding pixels). Here's how in PS.

Open PS and go to New, in the dialogue box change the units to Inches and enter 16 and 20 into the boxes which goes into the width and which goes into the height depends whether you want Landscape format or Portrait.

Here's the trick, the formula is 1mb per 1" on the longest side so you alter the resolution till you get a file size that is close to, but not below, in this case, 20Mb, this is shown in the bottom right hand corner, it will be something around 148 PPI.

Make sure the bit depth is set to 8 bit, and on the Advanced tab check that the output is sRGB and the pixels are square (these are the default, but it pays to check). Then click OK.

So we have our canvas defining the output size and the resolution we want, now we get PS to work our picture into it.

Go into bridge and highlight the file you want, then go to File>Place>In Photoshop. If it's a Raw file the Raw Converter will open adjust as you want and press OK.

PS will plop the picture centred in the canvas we just created with the Transform Tool already open. It's unlikely that the picture will match the aspect ratio so there will probably be bare canvas showing.

Hold down the Shift Key (to maintain aspect ratio) and click on any corner 'anchor point' and drag till the picture fills the canvas, this is where you decide which parts of the image to crop so it will fit the 20" X 16" paper. You can go back to this stage later if you want. You can also click and drag anywhere inside the bounding box and move the image around so you can get it exactly as you want it. When your happy hit Enter, or double click inside the bounding box.

If you need to go back to this to move the picture around around use the Ctrl + T shortcut which will re-open the Transform Tool.

You will notice that PS has opened it as a Smart Object this means that whatever you do it will go back to original file to get any data it needs, if you double click the thumbnail on the Levels Palette it will reopen the image file, if it's a Raw it will open it in the Raw Converter, any changes you make will be applied to the new canvas. Till you Flatten the image you are working totally losslessly, all the data you have is available.

If you double click the Zoom Tool (magnifying glass in the Tools Palette) it will give you a 100% view, this is exactly what the printer will lay down, you won't see any pixelation. If you hold down the Spacebar you can move around the image. Ctrl + 0 (the number zero) will give you full screen view and you can see the extremes again.

When your happy all that remains is to flatten the image Layer>Flatten Image and save. This is the file you send to your printer, there are limits, but you'll be surprised at how big you can go before there are more 'interpolated' pixels than actual data and the image degrades.

Chris