It would be best to leave it until after the flowers are gone. With citrus, it is best to only ever buy a pot slightly bigger - the next size up from the one you have, or the one above that as a maximum. No more that 13inches across the top I would say, just to play it safe until you get the care under wraps.

That potting mix should be fine, but I would still either sift out the fine matter with a sieve to improve drainage, or add some coarse material in - perlite, turface, pine bark chippings, coarse sand etc. Even the commercial citrus mixes, in my experience, are likely to compact over time, so add something into it.

Is tis your first tree? If so, read up all you can here on watering, feeding and where it should be located. Reading will be the thing that helps this tree live, trust me.

And don't worry if leaves drop off the tree when it is repotted. Just try to disturb the roots as little as possible (since this is your first repot) and give it time. Meyers usually drop a load of leaves after a repot if not careful lol.

On another note, all the flowers that turn to fruit on your tree won't all stay attached. I imagine a few will, but it is natural for citrus to drop 95% and more of the little fruitlets without them getting bigger. Its a natural way of shedding what it can't hold, so don't worry when fruit drops off.