# センサモードのコンボボックス
                self.sensorFormat = QComboBox()
                #self.sensorFormat.addItem("Default")
                self.sensorFormat.addItems([f'{x["formatfsensor"].format} {x["size"]}' for x in picam2s[camid].sensor_modes])
                # 最大の解像度(4608x2592)に設定
                # プレビューとキャプチャの画角違い防止のため固定
                self.sensorFormat.setCurrentIndex(2)
                self.sensorFormat.setEnabled(False)
                formLayout.addRow("センサーモード", self.sensorFormat)

...the program would no longer launch. I assume there are references to things in this code elsewhere in this script (or maybe other scripts) that I would also need to delete or modify.
I also would like to get rid of the "Image Size" (画像サイズ) item just below the sensor mode item.

        # 画像サイズ反映ボタン
        self.resButton = QPushButton("反映")
        self.resButton.setEnabled(False)
        # 画像サイズ反映ボタンクリック時の動作
        self.resButton.clicked.connect(self.on_resButton_clicked)
        
        # 画像の縦横サイズを変更時は反映ボタンを有効化
        self.imageWidth.valueChanged.connect(lambda: self.resButton.setEnabled(True))
        self.imageHeight.valueChanged.connect(lambda: self.resButton.setEnabled(True))
        
        # 画像サイズ
        resolution = QWidget()
        resHLayout = QHBoxLayout()
        resolution.setLayout(resHLayout)
        resHLayout.addWidget(self.imageWidth)
        resHLayout.addWidget(QLabel("x"), alignment=Qt.AlignHCenter)
        resHLayout.addWidget(self.imageHeight)
        resHLayout.addWidget(QLabel(" "), alignment=Qt.AlignHCenter)
        resHLayout.addWidget(self.resButton)
        formLayout.addRow("画像サイズ", resolution)

Getting rid of those two would make this window plenty usable and legible. (As it is, the "Rotation" control, just below the "Image size" control, is unusable with the window sized to fit my screen, because the button isn't visible.)

I've read bits of the original code and couldn't find anything suspicious.
So even when you resize the window manually, the result is not satisfying or what you expect?
In your video it looks like some comboBoxes (the elements the with a drop-down menu) are not displayed correctly?!

@Rachel_T said in Help changing someone else's code (default window size):

I believe there is some way to replace those numbers with percentages of available screen, such as "0.9."
Then there remains the problem of tweaking how other things get scaled, so that they are legible and useable on my little monitor

Yes it should be possible, in fact the actual code already uses some kind of scaling, because as you can see

self.resize(int( 1920*3/4 ), int( 1080*3/4) )

it uses 3/4 of 1920x1080 (which equals the size of FullHD resolution). I assume it's due to some scaling of their original screen.

Ah, while I am typing this, I see @J-Hilk has replied.

You can use code like he posted above to get your current screen size

(This part here)

screen = QApplication.primaryScreen()
screen_size = screen.size()
screen_width = screen_size.width()
screen_height = screen_size.height()

# this does not need to be 3/4...
# play around with different sizes until you get what you want
self.resize(int(screen_width * 3 / 4), int(screen_height * 3 / 4))

# this is where the window appears on your screen.
# topleft corner is the origin of the coordinate system ( 0 / 0 )
self.move((screen_width - self.width()) / 2, (screen_height - self.height()) / 2)

or you try one of

 self.showFullScreen()

or

  self.showMaximized()

instead of just self.show()

But I don't know for sure whether it will work (and set the correct size) on PyQt and your Raspberry.

Edit:

@J-Hilk The original code says PyQt5... Can't tell if there are possible issues with scaling and screen sizes (:
Could not find any trace of something around HighDPIScaling or so...

from PyQt6.QtWidgets import QApplication, QMainWindow, QLabel
from PyQt6.QtGui import QScreen, QFont

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        screen = QApplication.primaryScreen()
        screen_size = screen.size()
        screen_width = screen_size.width()
        screen_height = screen_size.height()

        self.resize(int(screen_width * 3 / 4), int(screen_height * 3 / 4))
        self.move((screen_width - self.width()) // 2, (screen_height - self.height()) // 2)

        font_size = int(screen_height * 0.02)
        font = QFont("Arial", font_size)

        self.setFont(font)


        label = QLabel("Welcome to Simple Book Capture!", self)
        label.setFont(font)
        label.move(50, 50)


        self.setWindowTitle('Simple Book Capture Ver.0.0.1')
        self.show()


app = QApplication([])
window = MainWindow()
app.exec()