Linen thread size is indicated by a numbering system. The first number is the size and the second is the ply. A high size number is a fine thread and a low size number is a heavy thread. A 100/3 size linen thread measures 0.0120 inches in diameter. A 120/2 would be a very fine thread used in stitching fine fabric and handkerchief weight fabric. A large heavy size would be 12/3 used for sturdier craft applications. The size of linen thread is not equal to other fibers that may use the same numbering system.
Linen thread comes waxed or unwaxed. Because linen thread has a tendency to shred, it is usually sold waxed. Waxed thread is difficult to dye, so if you wish to dye your own thread, purchase it unwaxed. Artisans and crafters wax their own thread with beeswax or microcrystalline wax. Waxed thread is stiffer and slightly sticky. Waxed thread will hold knots better and resists moisture.
A strong linen thread would have multiple plies and a low size number. Bookmakers use 18/3 and 12/3 linen. Leather craftsmen use linen cord in 5 or 7 ply. In the leather trade you may hear linen thread referred to as 5 cord or 7 cord. In some cases, only the ply is mentioned and not the thread size.
Don't confuse thread size with thread count. Thread count is the number of threads per square inch in sheets. A moderately high thread count of around 300 in sheets made from linen would be a high quality sheet that would be light and soft. Higher thread count requires finer thread size. Linen sheets become softer with washing and are valued as family heirlooms to be passed through generations.