Looks like it is 1/2 sheave. Does 3/8 make the most sense here then?
Also, the pictured line below is the topping lift. Does it look really oversized or is it just me?
Over time, the line ages and softens and can appear larger than the same size new line. Some lines seem more susceptible to this swelling, but the main reason is most sailors just leave them uncovered out in the elements their entire life.... which can be 20 or 30 years for the lazy owner. A 1/2 inch line of a 30 footer is unnecessary. If you want stronger line, don't get larger, get higher quality. working load, stretch and breaking strength are rope attributes that will be listed on the spool on which the line is stored. It is available in ANY marine catalogue that sells rope. If 3/8" is comfortable in your hand, works well on your winches and runs smoothly through your rig's blocks.... then match the line quality up to the preferred load. Use general rigging guides, or even the boat manufacturers ratings get this info. You'll often see this data in the owner's manual...but most rigging sites or chandlery's will have basic guides that consider a boat's size, weight and sail size to suggest line dimenstions.
This stuff is important for you to get familiar with if you plan to enjoy your sailboat more than just harbor cruising with sundowners. So start reading up on cordage... types, quality, purpose, etc. That info is all out there on the internet, go to sailmaker websites first, since they often have linked artlcles on these subjects. Another great source is the WEST MARINE advisor... check it out. Articles on everything imaginable.. No excuses now my friend. Good luck.
On what he said.