The best option would be to restructure your tables/relationships so that sizes are recorded in the same field, but in different records for both attendees and guests. A simple related table can be set up for that purpose.
Without that option, reporting the total count of sizes for each person attending--wheter invited or as guest, becomes difficult to impossible because the same record can contain size info for at least two different sizes.
I assume you mean a related table? If so, how do I ensure a relationship between the names on the list and their records in the size table?
To quote my previous post: "A simple related table can be set up for that purpose."
To get a clearer idea of the tables/relationships/layouts I might use for such a solution see this demo file that I use to illustrate many to many relationships: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
It is based on these tables/relationships:
A record in Contact_Event is created for each person slated to attend a given event. ID numbers from Events and Contacts are used to link in info about the attendee and the event they are expected to attend. A field in Contact_Event can be added and used to identify whether a given attendee was invited, is being brought as the guest of an invitee or is an employee/volunteer scheduled to work at that event. A field in Contacts can record the clothing size data.
And a self join between Contact_Event and a new Tutorial: What are Table Occurrences? of Contact_Event can be set up to link an invited individual to the person that will be bringing them to the event.