When you add a product in Linnworks you must use MM which is fine. But Bigcommerce only uses CM by default so when you upload a product through LinnLive 120mm turns in to 120cm. This makes it so you cannot use a dynamic postage table or show the dimensions of your item on the store.
You can change the dimensions manually in the store after you have uploaded a new product but if you update anything through LinnLive then the dimensions will automatically be put back to the MM figure.
It would make sense to add a auto conversion feature so they are uploaded as CM instead of MM saving a heck of a lot of time and effort making sure the dimensions are correct every time you upload or edit something.