If you need lists of the T-shirts sizes of volunteers at those scheduled jobs, first make a custom code in the Volunteers section. In this example, an available drop-down code field is used with S, M, L, and XL values, but a text field or number field [Small=1, Med=2, Large=3, XLarge=4] could work. Then set the size for each appropriate volunteer.
You will need some dexterity in Microsoft Word to customize a merge template. This is an example of a Word template customization that merges the volunteers' names and T-shirt sizes into a custom letter template. The template content is stripped, however, and replaced by a table [but the data source stays intact since we started with one of the Reporter's templates].
The Reporter's manual provides an overview of the process of starting with a template and revising it for a particular use. Help | Reverence Manual | Reference | Reports | Letters and Envelopes.
Customize the Sample New Volunteer Letter in the Reports | Volunteer Reporter | Letters and Envelopes templates.
This is what the template looks like after it is customized. In this example the <<Code15>> merge field matches the code customized as T-shirt Sizes in the Codes tab/Reference Codes in the Reporter. But yours will certainly be different.