Electronic – How to mirror a large component in Eagle

I don't think there's an easy way around this. I would open up the resistor-power.lbr library on its own, in the library editor, open up the first footprint (ACO1), change the major grid to 10mm, then go through each one and note those that are close. There's usually some kind of logic to the footprint names and numbers -- for this library, it appears the letter prefix denote a width class and the number denotes length. ^EDIT1: I went and did this, and none are 47mm wide.
^EDIT2: After looking at the datasheet, it appears it is 47mm long and only 9mm wide, which fits HPS947. (KH216-8, RS10-38-39, and RWM8X45 also seem to fit... sorta.)

In almost every PCB design you will be making your own parts. I find it useful to make a new EAGLE library for every project, and copy all used part into it. The easiest way to make a new part, if it's just a variation of another, is to copy symbols and possibly even footprints into a new layout, then edit them. For this part, you can copy the power-resistor.lbr > R symbol by opening it up as if you were going to edit it, selecting the whole thing with the Group function, then copying the group with the Copy function (click Copy button, right-click previously grouped symbol, select "Copy: Group". This puts the whole thing into your Paste Buffer:

• <-- (that button)

Open up a new library, example.lbr, then create a new symbol (Library > Symbol). Click the Insert Paste Buffer button ( ^-- that button). The copied symbol should come up, labels and all -- click on the crosshairs to line it up with the symbol anchor. I usually make my own footprints simply to remove all doubt in the library specs, but they can be copied using the same method. Another way to copy a part is to copy the entire library and rename it, then edit the entire thing. Sparkfun also has a pretty good tutorial on making parts.

Also, when I make my own parts, I fill out the caption so I won't have to browse the footprints directly in the future!

Use the mark command or the Mark button:

Then click the location you want to measure from. Unfortunately, the mark command doesn't accept coordinates; you have to click the location manually (and therefore, it must be on the current grid).

This will set a reference point, you'll see that a new box appears between the standard coordinates and the command line:

The first set of parentheses in this new area contains the rectangular offset from the mark to your current cursor location, as indicated by the R. In this example, I'm 0.1" to the right and 0.7" above the mark. The second set of parentheses is the offset in polar coordinates - I'm 0.71" away at an 81.87^o angle above the positive X axis.

This tool is available in both the schematic/symbol/component capture view and the layout/footprint editor.

For some repetitive tasks, it's much easier to set your grid to the distance which you want to space things out by and then move that group into place later. The [alt] key is your friend. This isn't a generic answer to your question, but it should be considered if you're doing a lot of measurement.