Guidance for using composite keys to identify rows
After thinking about this, I would revise your table structure just slightly.
First, revise your sheet-size table:  

Sheet_size
===========
Id
Length
Width
Thickness


Second, create a sheet-size/type relationship table:  

Sheet_size_type
================
Sheet_Type_Id
Sheet_Size_Id


Then, create the following constraints:


The primary key (and index) of Sheet_size should be the id column
There should be some sort of unique key (and index) enforced on the dimensions in Sheet_size.  Consider, are two sheets of dimensions (48, 96, .5) and (96, 48, .5) equal (that is, does the direction of the dimensions matter)?  This sort of problem may be difficult to enforce if through the use of the columns as part of the primary key, but becomes more manageable when using constraints and stored procedures.
The primary key (and index) of Sheet_size_type should use both of the foreign keys, starting with the one with the lower cardinality (probably sheet_type, given your example).  You may want an additional index in the other direction, but it might not be necessary.


This revision will save you database space (as a ratio of the number of sheet types using the same size), and shouldn't impact overhead too much.



There are other potential concerns about equality/uniqueness if you are using a float datatype, as the imprecision may trip you up unexpectedly.  You should consider whether or not a fixed-point type, to some given precision, would be more appropriate.