下面的例子主要是使用滑动窗口来控制字体的大小, 字体大小的设置通过QLabel().setFont(QFont('Arial', size))

QSliderDemo.py 

"""
滑块控件(QSlider)
"""
import sys
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import Qt

class SliderDemo(QWidget):
    def __init__(self):
        super(SliderDemo, self).__init__()
        self.initUI()

    def initUI(self):
        self.setWindowTitle('滑块控件演示')
        self.resize(300, 700)

        layout = QVBoxLayout()
        self.label = QLabel('你好 PyQt5')
        self.label.setAlignment(Qt.AlignCenter)

        layout.addWidget(self.label)

        self.slider = QSlider(Qt.Horizontal)
        #设置最小值
        self.slider.setMinimum(12)
        #设置最大值
        self.slider.setMaximum(48)

        #步长
        self.slider.setSingleStep(3)

        #设置当前值
        self.slider.setValue(18)

        #设置刻度的位置, 刻度在下方
        self.slider.setTickPosition(QSlider.TicksBelow)
        #设置刻度的间隔
        self.slider.setTickInterval(6)
        self.slider.valueChanged.connect(self.valueChange)

        self.slider1 = QSlider(Qt.Vertical)
        # 设置最小值
        self.slider1.setMinimum(10)
        # 设置最大值
        self.slider1.setMaximum(60)

        # 步长
        self.slider1.setSingleStep(5)

        # 设置当前值
        self.slider1.setValue(30)

        # 设置刻度的位置, 刻度在下方
        self.slider1.setTickPosition(QSlider.TicksLeft)
        # 设置刻度的间隔
        self.slider1.setTickInterval(2)
、　　　　#当刻度变化时, 将标签的字体进行设置
        self.slider1.valueChanged.connect(self.valueChange)
        layout.addWidget(self.slider)
        layout.addWidget(self.slider1)

        self.setLayout(layout)

    def valueChange(self):
        print("当前值: %s" % self.slider.value())
        size = self.sender().value()
        self.label.setFont(QFont('Arial', size))

if __name__ == "__main__":
    app = QApplication(sys.argv)

    main = SliderDemo()
    main.show()

    sys.exit(app.exec_())