"Size (Bytes)" is the column you want.
The only way to define your own columns is to write a shell extension (or get us to add new ones 
).
If you're using columns like Sub-Folder Count which force Opus to read all the directory listings below a folder, then the foldersize calculation will be done and displayed as part of the bargain. See this FAQ for more: Calculate Folder Sizes Automatically
Explorer will show a context menu for a completely locked file, but the menu won't let you do very much with the file. (Except maybe run Unlocker, which is a whole separate discussion.)