Are the Dimensions in inches or centimetres?

Dimensional Shipping’s dimensions are purely numeric; the shipping carrier determines the unit of measure, not the extension.

UPS, FedEx, Cerasis and USPS use inches for their unit of measure.

Are weights in pounds, grams…?

Magento’s weights are numerical when stored; the carrier determines the unit of weight, not the extension.

• UPS, FedEx and FedEx Freight use pounds by default, which can be changed in System > Configuration > Shipping Methods > (carrier name) > "Weight Unit"
• USPS and Cerasis use pounds and do not have settings that can be changed