This is what attributes are all about...

There are several methods to apply them and the following is a "basic" approach andwill requireing refining to get what you want exactly;

admin > catalog > option name manager > set a name i.e. Size
admin > catalog > option value manager > add values to size i.e. 5x7, 8x10, etc.
admin > catalog > attribute controller > select a product > apply/add size & a size
Repeat for other sizes that this is offered in
When doing this there are also selections for what this option will be priced at and here is where the fun begins. There are several methods as I stated. If the base product has a price then you can add incremental increases for the optional sizes. If you select 'this is priced by attributes' when you set the product up then the attributes will control the price.

A real good place to view and learn about these is by installing a separate ZenCart with it's own DB in say a folder named demo or?? and setupup with the demo DB information. This set of products has explanations for the product descriptions and covers the many types of using attributes.