Okay, before we start: you will not be able to change the size of characters, mining nodes, wsg flag etc. The size is only for some creatures, the transparency works for all creatures and characters. It is prolly not something you would use in your every day gameplay but it might be nice to use for editing mobs for zone edits etc.
So lets start, the first thing to do is to find the ID of the model you want to change. There are a couple ways to do this but i'm going to use WoW Model Viewers npcs.csv to find my model. [If you are going to change a character model you should pick your ID number from the list at the bottom instead of looking in npcs.csv]
Open npcs.csv in firefox by just dragging it into the browser (you can use notepad aswell but firefox is sexier), press F3 (or ctrl+f) and enter the name of the creature you want to edit, I'm going to use a Swine from durotar.
You might not get the right creature right away since many have similar names, just press Next until you get the one you want.
Now that you have found your creature you need to write down or remember the ID (in my case 10015), it is the second number from the left.
Use My WarCraftStudio to look for and extract CreatureDisplayInfo.dbc from patch-enGB-2.mpq or patch-enGB.mpq (if patch-enGB-2.mpq has it you should use that one, they are prolly named enUS if you have the US version). Open up CreatureDisplayInfo.dbc with a dbc editor (if you dont have one you should check the model editing programs sticky), scroll down until you get to the number you have written down.
What we are going to edit is Field4 and Field5, field4 is size and field5 is transparency, i want my swine both larger and more transparent so i will edit both.
I'm increasing my size to 1060000000, you can experiment with this a bit to get the hang of how the size changes.
I'm putting my transparency to 50, this can be any number from 0 to 255, 0 being invisible and 255 being no transparent at all.
When you are done just save the dbc.
Open MWCS and make a new mpq, put it in your Data folder and name it patch-#.mpq (#= any number from 3-9 or letter from a-z).
add file to archive, browse to your dbc file, add it and change the name to DBFilesClient\CreatureDisplayInfo.dbc
Now you are done! time to get ingame to see if the size change worked.
well mine didnt, only some creatures allow you to change the size, the rest is set by the server. you can still see the transparency tho :P
Character ID's list:
51 -orcmale
52 -orcfemale
57 -UDmale
58 -UDfemale
59 -taurenmale
60 -taurenfemale
1478 -trollmale
1479 -trollfemale
15476 -bloodelfmale
15475 -bloodelffemale
49 -humanmale
50 -humanfemale
53 -dwarfmale
54 -dwarffemale
55 -nightelfmale
56 -nightelffemale
1563 -gnomemale
1564 -gnomefemale
16125 -draeneimale
16126 -draeneifemale