There are several lengthy discussion on various types of sails and sailmakers over in the "Sails" part of the forum. Have a look.
You're not giving us enough the necessary details to help you with your question on the mainsheet block, so we would have to guess. (Not all DaySailers are rigged the same, far from it).
Do you have center boom sheeting (mainsheet goes up and down to the middle of the boom but not aft to a traveler)?
Because I can't easily picture how you would use a double block with a traveler setup, I'm assuming you have a very minimal center-boom sheeting: up-down-and-up, with a single block on each end?. Correct?
What kind of double block were you going to fit: one with parallel sheaves, or a fiddle block? (I think the advantage of the latter is that the pull is more inline, also, the sheave over which the sheet has to travel a longer distance is larger).
Also, if you are making changes, I would strongly recommend that you incorporate a ratchet block in that setup, if possible.
As for the sheave diameter 39mm is not too big.
Finally, length of sheets. I think it is best to measure that on your boat, and not simply assume that a standard measurement is correct, only to find out it's a few inches short.
Mainsheet: the mainsheet needs to allow the boom to go forward until it touches the shrouds. For center boom sheeting that creates a straight line distance between the boom attachment and the main-sheet swivel. You measure that and multiply by the number of parts in your purchase and add one foot for a tail. If you are using double blocks that should be accurate, if fiddle blocks, it might be slightly generous, but better to trim the last foot than be a foot short.
Jibsheet: the jibsheet needs to allow the clew of the jib to extend from the forestay at 90 degrees and from there, be able to reach to the other(!) side of the boat, through fairleads and cleat and have a bit of a tail for grabbing. The range of variations for jib sheet rigging is a bit less, but cleat location isn't the same on all boats, so rather than getting a "standard" value where you don't know what rigging was assumed, you're better off measuring it.
(I measured my spinnaker sheets, and came out a bit longer than some "standard" recommendations; they ended up being a bit tight - could have used another 6". Lesson learned: buy a bit extra, unless you are replacing an existing line that you can measure precisely).
If you haven't already, I suggest you have a look at the thread "
Rope for various lines" where we've been discussing the advantages of various types of rope to use for various sheets.