While 128MB is a sufficient size for most people, no doubt it is wasting RAM for some, and for others, it is too small. Below is the procedure to change it, and to test that change in relative safety.
You may or may not wish to read up on rescue console, just in case.
The core of this is that we are going to edit fastinit, which appears to be part binary, part text. Lucky for us, we're editing the text part.
cd /sbin sudo cp fastinit fastinit.orig
fastinit.orig should never be changed - it's your 'Oh crap, I just edited the actual fastinit' file
sudo cp fastinit fastinit.256tmp
fastinit.256tmp is my name for my 256MB tmp size. You can call it what you like.
sudo khexedit fastinit.256tmp
set txt instead of hex via the drop down
Find 128m with the find option (offset 172c for me)
Change it to 256m by clicking in either the top left or top right pane and typing (hex in the left, or if you just want it done, regular typing in the right
Hold down/press repeatedly (I held down, and it worked) F9 on the very initial BIOS screen
Select the 2nd line, press “e”
get to the end of the line, add a space and then:
press b to boot
It should work fine. At this point, if you just boot normally, you get the default tmp size. If you press F9 and follow the instructions after that point, you get your custom size.