If the container is too big for the parachute it is possible that a pin-closed rig will have very low pin tension. Too large is rarely a problem with Velcro-closed rigs.
Consolidated Rigging manufactures the Perigee Containers in 8 different sizes ( Perigee Pro and Perigee II are available in 7 sizes). We generally size a container for a comfortable fit around the parachute. A neat packer or a person packing in humid conditions will find this results in a rig that is secure and easy to close. However, if your BASE pack jobs are not so neat, or if you often jump in dry (desert) conditions you may prefer a larger container size. If this is the case, select "soft" on the order form. If you are not sure, select "standard" on the order form.
You can provide information regarding your prefered fit in the comments field at the bottom of the form. Include things such as whether you do aerials, if you will be using the rig for more than one size parachute etc.
If you have any questions please feel free to ask.