What size does buy trade when I give no size?
The usual amount, set where the script runs.
A script that says buy and nothing more is a reasonable script. It means “the usual amount”, and the usual amount belongs to wherever the script is run, not to the script. In the Terminal that is the Lot size in the backtest panel and the Trade size when you deploy. When nothing is set anywhere, it is the smallest lot the market deals in.
A script that names a size: or a risk: is never overruled.