How to change the width of dialog in android compose

Use can define a custom AlertDialog using the constructor with text,title and buttons parameters and applying a size (for example with the Modifier.size) and overriding the default behaviour with usePlatformDefaultWidth = false :

AlertDialog(
    onDismissRequest = { /*TODO*/ },
    title = {
        Text(text = "Title")
    },
    text = {
        Text(
            "This area typically contains the supportive text " +
                    "which presents the details regarding the Dialog's purpose."
        )
    },
    buttons = {},
    properties = DialogProperties(
        usePlatformDefaultWidth = false
    ),
    modifier = Modifier.size(200.dp,250.dp)
)

If you want to use a constant width in all your project you can create a dialog with customized width as follows

@Composable
fun MyCustomDialog(
    onDismissRequest: () -> Unit,
    properties: DialogProperties = DialogProperties(),
    content: @Composable () -> Unit
) {
    Dialog(
        onDismissRequest = onDismissRequest,
        // We are copying the passed properties 
        // then setting usePlatformDefaultWidth to false
        properties = properties.let {
            DialogProperties(
                dismissOnBackPress = it.dismissOnBackPress,
                dismissOnClickOutside = it.dismissOnClickOutside,
                securePolicy = it.securePolicy,
                usePlatformDefaultWidth = false
            )
        },
        content = {
            Surface(
                color = Color.Transparent,
                modifier = Modifier.width(250.dp), // Customize your width here
                content = content
            )
        }
    )
}