David Spencer has put up some good resources on how to install Slackware ARM aka ARMedSlack into Raspberry Pi devices. ARMedSlack is developed by Stuart Winter of the Slackware Team and now AlienBOB is joining up to help the work of this porting. I'm hoping to see ARMedSlack releases in the future for ARM architecture.
David has provided additional packages such as XFCE 4.10, -Current ISO, XOrg-1.12 patches, and raspi-hacks.
You will need at least 2 GB card for default installation or 8 GB card for full installation. Considering that 8 GB card is not that expensive right now, i think it's the best choice.
Follow Stuart Winter's Twitter account for ARMedSlack updates