ekmPowershop supports variants such as sizes and colors, allowing you to manage the items as a group. People buying on your ekmPowershop store will be able to select which variant they want from a drop-down. You can also create variants in Brightpearl and as long as each variant has the same SKU in both ekmPowershop and Brightpearl inventory and orders will work fine.
Each variant in ekmPowershop and in Brightpearl has their own separate product record, and so they connect as usual by SKU. Each item has its own inventory level managed by Brightpearl as for any other single product.
Note that Brightpearl allows a maximum of 4 dimensions for a product, for example, a product might have a choice of size, color, material and pattern, but ekmPowershop allow you to add more than 4 variant choices to a single product. If you have products with more than 4 dimensions on ekmPowershop you can continue to manage them in this way, however, you will need to separate the products into separate groups in Brightpearl by adding dimensions into the product name.