I want to edit a few units...for example: making the battlecruiser 2-3 times bigger, and changing the price, HP, build time and power to match.

All this can be done trough the data editor, right? Or is this only for when you make new maps?

I haven't toyed with mods yet (I don't know if they are loaded from start-up or loaded on a per-map basis), but that could be an idea. Granted, a modded Starcraft may not be able to load the Campaign file if there are any version checksums going on.

As far as just through the data editor without a mod, then no, I believe you would be out of luck. If SC2 is anything like SC1, then the data is loaded in this order:

Maindata.mpq > Expansiondata.mpq > Patchdata.mpq > Mapdata.mpq.

Meaning that, unless the map is loaded, it wont take any effect. As each level of the campaign is a map (and so are some of the cutscenes, and even the stages upon the hyperion), any map with these changes wouldn't have any effect on the campaign. I could be wrong though.

I am not sure about your question.

You can either work on a new map or a new mod to achieve your goal: the only difference between those is the fact that in a map, you can place units, terrain, and etc.
From the mod or the map, you can open the Data Editor and search for the Battlecruiser unit in the "Units" tab to modify the values such as "Cost" and such.

You guys confused me more than the OP. I don't know what you guys are talking about, but the simple answer is yes, all that can be done through the data editor.

To make a Battlecruiser bigger, find the Battlecruiser unit. Under "Models," click "Battlecruiser." Scroll down until you find the "Scale Minimum" and "Scale Maximum" section. Change the "X," "Y," and "Z" of both to whatever scale you want. It defaults at 0.8 for all values, and just make it 1.6 for 2x the size and so on.

To change the price, go back to the Battlecruiser unit, and click the "Cost" tab. Under "Cost" click on "Minerals," "Vespene," etc. to modify how many minerals and gas it will cost.

To change the HP, click the "Stats" tab, and go down to "Life Maximum" and "Life Starting Amount." Change these to whatever you want the HP to be. You can also mess around with life regeneration here or give it shields, and so on.

For the most part, look under the tabs to change all sorts of stats and such.

The formulation he used never lead me to think he wanted us to show him where to change the size of the BC or anything. It looked as if his question was "All this can be done trough the data editor, right? Or is this only for when you make new maps?", and the rest was just an introduction. ;o

He's stating what he wants to do, and then asks if he can use the data editor to do it.

So the easy answer would have been "yes," rather than analyze what else he could have meant.

I thought he wished to change this in the actual Single-Player campaign, and didn't really see anything to say different, considering that the battlecruiser is not a Campaign specific unit.

Since it isn't a unit specific to the included campaign, I figured he was asking about editing units for a single-player campaign he is making.