That is a great question, and there is not a one size fits all answer. There are many factors and variables involved to determine that answer. First there is risk appetite, how much are you willing to risk or lose on each trade that you take? Also known as leverage.
Next, is how often do you plan on making or taking trades as well?
Lastly, there is the most important piece, your edge. Your edge in most cases determines your leverage and also your frequency.
Many times people come into the markets Forex or penny stocks seems to be the hot ones that bait in novice traders with the gambler mentalities. But these items apply to any trading system or tradable market that exist. You see them trying to scalp EUR/USD or some other exotic FX pair on a 15m or smaller time frame chart without factoring in the spread of each Forex Pair, this can eat your account alive if you do not keep an eye on the spread and how wide it is when entering and also when managing your open forex trades. The brokers love to widen spreads during the off hours which can lead to unnecessary and in most cases avoidable losses. This is where size plays a role in your trading process and frequency as well.
Trading is a numbers game and you need the law of large numbers to work in your favor so over time your edge kicks in and you end up ahead. In order for this to play out you need to know your frequency how often your edge shows up in the markets and also have your leverage in check so you do not end up blowing up your account.