"用大字体书写" can be translated into English as "write in large font" or "use a large font size when writing".
The first expression, "write in large font," is more concise and commonly used in contexts where the focus is on the visual appearance of the text, such as in design, signage, or presentations.
The second expression, "use a large font size when writing," provides a bit more detail about the action, specifying that the large size should be applied during the writing process. This might be more appropriate in technical or instructional contexts where clarity is essential.
In most casual or everyday situations, "write in large font" would suffice.