you have 3 tables e.g Design.colour and Size.
Make sure for a single design multiple colours are there and for a single colour multiple size are there.
So Place 3 global fields on the layout and Do following steps.
1>Create a valuelist that keeps all the colors and attached it to the Design global field.
2>Create another value list from the relationship e.g designglobal fields to colorDesignId which gives only the related designs color.
3>Also similar process create anothe valuelist for size from the relationship e.g ColorGlobalfield to sizecolorID and it will give the realted colours size.
I think it will help you for filtering depends upon the previous value.
1) this is called a conditional value list and stringing 2 or more together like this is something I call a Hierarchical Conditional Value List. Here are some links on the concept:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.
2) This thread on the two basic methods for using the info entered into a field to look up data from a related table (you'll want to copy data from a related table) should help: Auto Fill
Thanks for the fast response, I am currently working on another part of the database but will be back round to this either later this afternoon or tomorrow.
Thanks again for taking the time to read such a lengthy post and offer help.