The strongest argument for a small first deposit has nothing to do with markets. You are testing a process — signup, verification, funding, a first position and, most importantly, a withdrawal — and you want that test to cost as little as possible.
Run the full loop with the minimum amount. Deposit, wait, withdraw part of it, and watch how long the money takes to return and whether it comes back to the method you used. A platform that handles a small withdrawal cleanly is one worth scaling into.
Only after that round trip does it make sense to think about size, and even then in steps rather than one move. A bigger deposit does not make a strategy work better; it only makes the same outcome larger in both directions.
Why the first deposit is the one to think about
The first deposit sets the habit. An amount chosen for comfort tends to be followed by calm decisions; an amount chosen because it felt like the maximum possible tends to be followed by decisions made under pressure.
A workable starting point
Money you would not need back within a year, in an amount whose loss would be annoying rather than damaging. That is a personal number, and only you can set it.
Adding to it later
Topping up a balance you already understand is a far better position than starting large and learning afterwards.
Questions worth asking before you send anything
How do I withdraw, and to where? What gets deducted, and by whom? Who do I contact if something looks wrong? A service that answers all three clearly and in writing is behaving the way it should.
Investing involves risk, including the possible loss of some or all of the capital you put in. The value of investments can go down as well as up, and you may get back less than you originally invested. Do not invest money you cannot afford to lose.