My Euro securities account does not accrued interest even though it is negative. I will wait for a month to see if I really get charged with Euro interest.
I have compared my account with my wife's which happens to have negative Euro balance since it has not been granted for Euro box trades. My wife's account gets Euro interest accrued on a daily basis.
I would like to see what will happen if the futures account gets more than Euro50k deposit. Will it get negative interest rate?
It looks like selling a bigger amount ESTX50 box will save more on the margin capacity of an account. ie Big box is not linearly bigger in terms of margin requirement compared to a small box.
Is this a correct assumption?
You are having a weird situation in the first place, as EURO Stoxx 50 (non-futures) index options got credited to your futures account, if what you are reporting is indeed correct. But please let us know if you get charged anyways. If not, I wouldn't apply your situation to any "normal" U.S. account. I did get charged interest in the securities segment.

Bigger is better for the reasons you cited. You can trick The Algorithm by putting in a GTC order opposite your position that is so far out of market that it will never get hit (and if it did you'd have an instant profit from it) and it uses those as the best bid/ask as it's too stupid to realize the order came from you. Or better yet, use any broker besides IB because now that you figured out a way to get cheap margin there's no reason to put up with their horrible customer service and arbitrary The Algorithm that apparently runs the company with an iron fist.Selling bigger box vs smaller box, which one is better?
It seems that selling bigger box is better as it saves on margin requirement and option transaction fees. How about other aspects like the likelihood of the box being broken up by IBKR liquidation algo and option costs that we have to pay and we will receive.
Bigger is better for the reasons you cited. You can trick The Algorithm by putting in a GTC order opposite your position that is so far out of market that it will never get hit (and if it did you'd have an instant profit from it) and it uses those as the best bid/ask as it's too stupid to realize the order came from you. Or better yet, use any broker besides IB because now that you figured out a way to get cheap margin there's no reason to put up with their horrible customer service and arbitrary The Algorithm that apparently runs the company with an iron fist.
I've stopped do it because I'm no longer using margin. I usually went out a couple months when I was doing it, but not sure of current market conditions.How far out do you trade the box for best liquidity /fills?