Yes you are allowed to pick your bet size but if there is a min deposit then chances are the gamblers willl gamble most of their initial deposit.
Most new gamblers basically all follow the same path.
They deposit $1 and play 1 cent bets on martingale until they eventually go bust with 20 reds in a row.
If they are required to deposit at least $20 or so, then they will most likely go bust with the entire $20 and not the $1.
It's basically a psychological thing. If the money is there they will gamble it. If they need to wait then most likely they will come to their senses and not gamble on fear and greed.
Like bitsler support said that they do not have any min deposit amount but
do you think you are going to deposit 0.001 btc mean while the fee is almost the same? I do not think anyone willing to do this. They deposit the amount and they should calculate it as well. So $1 or $20 will be just the same if you are going to gamble. You will consider this thing as lose from the start you make a deposit
But only luck that can make you get some profit on gambling site
I'm not really sure what wallet you use or when was the last btc transaction you made, but recently btc transaction fee is not that high. I spent less than 5000 satoshi for btc transactions in the last few weeks. So saying deposit 0.001 while the fee is almost the same is exaggerated.