You could use the FontTypeMetrics method of MagickImage to calculate the size of text you are drawing. Below is a simple example for when you are trying to write a single word:
string word = "Word!";
using (MagickImage image = new MagickImage(Color.Yellow, 100, 100))
{
image.FillColor = Color.Purple;
image.Font = "Garamond";
image.Density = new MagickGeometry(72, 72);
image.FontPointsize = 1;
TypeMetric typeMetric = image.FontTypeMetrics(word);
while (typeMetric.TextWidth < image.Width)
{
image.FontPointsize++;
typeMetric = image.FontTypeMetrics(word);
}
image.FontPointsize--;
image.Annotate(word, Gravity.Center);
image.Write("output.png");
}