Question 50% width

how can i make an item take up 50% of the screen's width using xml? more specifically a checkbox.

thanks
Phone: Google Nexus One
Rom: CyanogenMod' Nightly
Theme: None
Recovery: RA 1.7.0.1
Radio: 32.41.00.32H_5.08.00.04
SD Card: 8GB Class 4 - ~7GB Fat32 / ~1.5gb Ext4