There is limit of transaction data size?

For example, I mined continuously some alt-coin,

and when I try to send some big amount of coin (over millions), -qt wallet program stopped and after some while,

it spit out error message that transaction size is too big, so can't send coin.

After I lower the amount of coin to send, sending executed.

So this means, for gathering million coins from too many 1000 coins mined reward,

-> transaction data size become big, can't send coin?

Then, how much size is limit and where can I revise it at source-code?

So from what I experienced, I infer like this,
----------------
A wallet have 10, 100 thousand mined record. Mine reward is 1,000 coin.

1. Try send 500,000 coins,
A wallet should gather 500 mined transactions from its own wallet, and then should sum them up and make as one pile and send to address?

-> I don't know how much filesize this become, how much this become? How much size increase per one transaction record?

2. Try send 5,000,000 coins,
A wallet should gather 5,000 mined transactions from its own wallet, and then should sum them up and make as one pile and send to address?

-> This occured error that file size is too big, so can't send coin.