you basically asked half of the money management side of forex in a few lines.
I try to explain concisely but for more info you have to check out money management sections.
In general 1 to 2 percent of your equity is a safe portion of your account to risk in every trade but with smaller accounts, you can increase that to 5%.
with your $100 account, assuming your sl is 20 pips on eur/usd , your lot size could be something between 0.02 to 0.03. 0.01 lots are worth 10 cents so you risk $4 to $6 or 4% to 6% of your account each time. As the account grows, you have to modify the numbers based on that.
regarding leverage, the bigger account the lower leverage and vice versa. with a $100 account go for 1:400 or higher.