How about 5ft X 4ft or even larger?
The trick is to force PS to apply the minimum amount of interpolation, if force is the correct word. Here's how I do it
Open PS and got to File>New
In the dialogue box enter the width and height using inches as your units
In the Resolution box select the resolution that gives you the file size (lower right) that is 1Meg per Inch on the longest side, this will be a lot lower than what you think. This formulae of 1 Meg per Inch is assuming your picture is being printed at 300DPI.
Click OK
Go into Bridge and highlight the picture you want to print.
Go to File>Place>In Photoshop
PS will plonk your pic in the centre of the canvas with a transform tool already selected.
Hold down the Shift Key (to maintain Aspect Ratio) and grab a top corner 'Anchor Point' and drag till the picture fills the upper frame. Do the same for the lower half.
This method opens the image as a smart object so if it is a Raw file double clicking the layer icon will re-open the Raw converter. Any filter will be a Smart Filter and can be adjusted such as the High Pass Filter for High Pass Sharpening.
You can move the picture around using the Move Tool (V on the keyboard) to get exactly the framing you want. Ctrl + T will call up the Transform Tool again should you need it. In short everything is editable losslessly at this point.
When your happy Flatten the Layers and save, this is the file you send to your printer and when the interpolation is applied.
Double clicking the Zoom Tool will give you a 100% view which will show you how the printer will lay down the ink.
PS is 'inventing' data of course, this method uses the minimum interpolation required to fill the matrix the printer is laying down. There are obvious limits, but you can definately get commercial prints bigger than the 20" X 16" which is often quoted as the largest 'safe' size.
There are a couple of caveats, first you need to shoot Raw, second the image should be good 'in camera' no blown out highlights etc.. Any fault in the picture will. literally, be magnified.
Chris