How do I remove the checkmark?

It's a flag indicating that the size was calculated automatically. Simply resize the popup window if you want a different size. Drag the right bottom corner to size the window as you want. After that it will remember the size you selected and use that one by default. You can resize it again whenever you want.
Shall you decide to let SQL Assistant calculate optimal window size, select the Automatic window Sizing

[quote=“SysOp”]这是一个标志，表示大小是自动计算的。如果您想要不同的大小，只需调整弹出窗口的大小即可。拖动右下角以根据需要调整窗口大小。之后，它会记住您选择的尺寸并默认使用该尺寸。您可以随时再次调整其大小。
是否决定让 SQL Assistant 计算最佳窗口大小，请选择“自动窗口大小调整[/quote]

thank you!!!