Often recommendations for block selection goes with the size of your boat. But be careful. In fact, you need to determine its working load conditions. Working load is fonction of the sail dimensions and wind conditions, not boat size.

When the say "Boat size", they are usually refering to the typical sloop sail plan. If you have a ketch, a yawl or a schooner, you definitely need to measure the actual sail dimensions.

Thus, first step is to measure luff length (P) and the Foot length (E) of mainsheet in meters. Note that web site like Sailboat Data provide generic dimensions on many production sailboat.

Secondly, measure the distance between the far aft end of your boom to your mainsheet attachment (x) point in meters.

Also, determine under which wind conditions in knots (W) you will pick your first reef. Sportier individuals will probably start at 30 kntos others cruisers at 20 knots. Make sure your standing rigging and your sailcloth will respond to those conditions too. 

Mainsheet load formula (Metric)

(P) Foot length of main in meters
(E) Luff length of the main in meters
(W) Wind speed in knots
(x) Length from aft end of boom to mainsheet attachment in meters

Sail plan Dimensions