How to set custom default size for new XFCE terminal windows?

Open ~/.config/xfce4/terminal/terminalrc (in older versions ~/.config/Terminal/terminalrc) and change the line with MiscDefaultGeometry to e.g. MiscDefaultGeometry=X*Y*Z with your default size.

@palmic is correct, in xfce 4.10 the ~/.config/Terminal/terminalrc is ignored. Instead, configure .config/xfce4/terminal/terminalrc. It has the same syntax.

More information in docs for xfce4-terminal(1) in ENVIRONMENT section.

The man page for xfce4-terminal specifies using the --geometry option to set the size and placement of a terminal.

This flag uses the options "-geometry WIDTHxHEIGHT+XOFF+YOFF (where WIDTH, HEIGHT, XOFF, and YOFF are numbers) for specifying a preferred size and location for this application's main window."

Refer to the "Geometry Specifications" section of the man page for X (man X).

Here are several examples of xterm placement:

1. Place a short and wide terminal window on upper left-hand side of the display:

   xfce4-terminal --geometry 140x20+50+50

2. Place a square terminal window in the lower left-hand corner of the display:

   xfce4-terminal --geometry 100x40+40+500

3. Place a square terminal window in the upper right-hand corner of the display:

   xfce4-terminal --geometry 100x40+1500+40

4. Place a short and wide terminal window on lower right-hand side of the display:

   xfce4-terminal --geometry 140x20+1500+500

How to make this setting permanent:

"copy the terminal launcher (terminal's .desktop file) from /usr/share/applications/ to ~/.local/share/applications/ and edit the Exec field accordingly" as mentioned in the answer here.