If "text mode" means as shown on a virtual terminal (typically accessed by Ctrl+Alt+F1 through 6 or Shift+Ctrl+Alt+F1 through 6) then passing a vga= kernel parameter from the boot manager (LILO or GRUB) according to the following table may do what you want:
Code:
# +----------------------------------------------------+
# Colours | 640x480 800x600 1024x768 1280x1024
# +-------+--------------------------------------------+
# 256 | 0x301=769 0x303=771 0x305=773 0x307=775
# 32K | 0x310=784 0x313=787 0x316=790 0x319=793
# 64K | 0x311=785 0x314=788 0x317=791 0x31A=794
# 16M | 0x312=786 0x315=789 0x318=792 0x31B=795
# +----------------------------------------------------+
If "text mode" means in a terminal emulator running under X then configuring the terminal emulator will do what you want. Details will be specific to the terminal emulator.