or is it more like
how small should i let the amounts in coin control get before i make new wallet.dat file
cheers
This is completely up to you. You lose no staking interest as long as the pieces eventually stake. You can always take a look on the blockchain to see what amounts are splitting. If you see only 2 transactions for a block, the amount seen is what split. So if you have a piece 1500 kobo big and another 500 kobo big, it'll take the 500 kobo roughly three times longer to stake because of it's smaller size. If you gather pieces up, sending them as one bigger piece back to yourself, you lose their accrued interest. They will stake faster that way, but at a loss in two ways. First, the initial pieces age; every movement of coins starts with 0 aged coins. Second, bigger pieces split faster, having you earn only a tiny bit of interest on the new large piece, only to start the cycle over. If you want the best of both worlds, you could gather coins then shut the wallet down for a time period, or better yet, change wallets to keep supporting the network. I think it's safe to just wait the blocks out as long as they aren't too tiny. My smallest is 358 kobo with 31038 confirms(from 2/19)--maybe it will stake in a full months time.
A new wallet file won't change anything really. Wait until it gets over 10mb, to even give a seconds thought to it.