The original post caused a bit of confusion because I didn't mention it affected dimensions like 8.5″which are not whole numbers. I've italicised my extra text.

In London it's the rule that you wait ages for your bus, and just as the rain stops they come along in pairs, or threes, one after the other. Well, three times this week I'm been asked how to do an upres in Lightroom so I thought I'd post a quick explanation and a preset.

All you need to do is exploit the Constrain Size settings in the Export dialog. Tick Constrain Size, and set both limits to the size you want - setting both is to allow for a mixed export of portrait or landscape orientation images.

Unfortunately because of (what must be) a bug you can't directly specify the physical size in inches or centimetres if it isn't a whole number - you have to work out the exact number of pixels. So for example I wanted to resize to A4 paper at 300dpi - 29.7cm which the dialog box rounds up to 30. That's 3508 pixels (rather than resort to Excel, I simply created a new A4 document in Photoshop).

Once you've worked out the settings, save them as a preset such as my A4 TIF to Photoshop template. On Windows this goes in C:Documents and SettingsUSERNAMEApplication DataAdobeLightroomExport Presets, while on Mac they're in LibraryApplication SupportAdobeLightroomExport Presets. Create the Export Presets folder if it doesn't exist.

If you're interested - I feel I should be but confess I'm not - Lightroom perform the interpolation using the Lanczos kernel method (please come back after you've proved the equation). That's the same resampling technique as in Adobe Camera Raw. It seems more worthwhile to ask whether it's better to do the resizing in Lightroom rather than in Photoshop, and I'm undecided on the matter. If anything, I'd prefer the latter because you can see the results and immediately reverse the operation.