简短版本：

splitter->setSizes({1000, 2000}); // 这个值至少跟像素值设置的一样大，或者更大，例如x10倍

详细版本：
setSizes 官方介绍如下：

Sets the child widgets’ respective sizes to the values given in the list.
If the splitter is horizontal, the values set the width of each widget in pixels, from left to right. If the splitter is vertical, the height of each widget is set, from top to bottom.
Extra values in the list are ignored. If list contains too few values, the result is undefined, but the program will still be well-behaved.
The overall size of the splitter widget is not affected. Instead, any additional/missing space is distributed amongst the widgets according to the relative weight of the sizes.
If you specify a size of 0, the widget will be invisible. The size policies of the widgets are preserved. That is, a value smaller than the minimal size hint of the respective widget will be replaced by the value of the hint.

重点如下：

splitter大小不会变化，比设置的值多的和少的空间将会按照sizes之间的比例分配
size policy保留，且widget至少有minmum size的最小值大小

网上说的setStetch方法对我不管用（左边使用QTreeWidget，右边是QWidget），并且会受到sizePolicy和minimum size影响，因此我并不推荐使用。