Thanks for the question! You can change the default drawn by changing the Battle Start Actions and Turn Start Actions parameters. For example, let's say your Hand Size is determined by Variable 7 called MaxHandSize. You can edit the Battle Start Actions parameter to say: "Shuffle Deck, Draw \\v[7]" and the Turn Start Actions to say: "Discard Until 0, Draw \\v[7]". The number of cards drawn in each case will be based on the value of Variable 7.
If you want to tie this to Equipment, you can use the IsiahEquipCommonEvents plugin and have one Common Event for equipping the Item and one for unequipping the Item specified in the Equipment's notetags. The Common Events should manipulate that MaxHandSize variable value to give you the intended effect.
Hope some of this helps. Let us know if you have any other questions or wish to troubleshoot further :)