I have two spreadsheets, one with data listed against a product identified by the style (sku), the second spreadsheet has the different product colour and size options for each sku. I want to merge the two spreadsheets together so that I have all of the colour and size variations for each sku as well as all of the details that are matched to the sku i.e. long description, product name, catalogue etc... I am trying to do this in excel
You need to Consolidate Data. Excel can merge data from different spreadsheets (worksheets) or different files. Another thing you can do is use a Pivot Table.
For Consolidation, read this support article from Microsoft.
For Pivot Tables, you can read this question in Stack Overflow which has a good example and also has another alternative (by using SQL) to accomplish what you need.
You can use the vlookup() function in your second worksheet to merge its data with the data from your first sheet. You can google for more information about the syntax of the vlookup function or find more details here: http://www.ablebits.com/office-addins-blog/2011/12/14/lookup-excel-functions-vlookup/
If it is a one-time task and you don't have time to learn vlookup(), install the Merge Tables Wizard add-in (it is commercial, but a fully-functional 15-day trial version is available), you can download it here: http://www.ablebits.com/excel-lookup-tables/index.php
If you opt for the Merge Tables wizard, be sure to choose your SECOND worksheet with colour and size variations as the Main table. Merging 2 tables will take you about 5 min with the add-in installation.