It's hard to tell how much space you've got, about two 9v batteries would be nice.
Looking at the pictures it seems... enough for at least a few coincells, 6 leds, 6 resistors and a Mini, Pro Mini, Micro or Nano.
The first two boards are the smallest, but need an external USB-breakout board to program them.
Unfortunately the product pages of these boards don't specify size, each pin on a connector is placed 0.1 inch from the next though.
Next part, how would you like to control/configure your leech ?
You could for example use a power switch to start the (standard programmed) routine, which requires access to a pc when you want to change settings. Looking at the original leech, you may also be able to include some switches/potentiometers to adjust settings without the need of a PC.
Most of the functions needed are described in the first two sections of the built-in tutorial of Arduinos IDE by the way, it shouldn't be too difficult to get a working proto(n)-type 