Yes you can increase or decrease lot size. eg. if you start with 1000 then the lot size is set at 0.01 and when your equity rises to 2000 then you can increase to 0.02. As the equity in your account increases you can increase your lot size.
If you withdraw funds from your account then it is important to decrease your lot size too. This ensures your capital is managed with the appropriate leverage and you are adhering to good risk management guidelines.
Risk management is an important pillar in trading and its important that one understands this. Alot of people lose money with forex trading because they have over leveraged their trading account and are risking too much of their capital on trades.
I've seen this a number of times now when people send me their screen shots of their MT4 app and its very upsetting to see this. When you don't adhere to proper risk parameters many find themselves where their trading accounts just bleed money. If you are risk averse then this is also an important pillar to learn.
I serve people who want their money to work harder for them instead of it sitting in the bank.