font and size options blank on selecting all despite uniform formatting

Dear @kubidushu,

Welcome to Microsoft Q&A Forum! 

Based on your description and provided images, I have run a few tests on my side and I want to share my insights and recommended method for you in this case: 

• When I do the same as you in Microsoft Word application on my MacBook, I have the same issue if my line counts about over 49.  

• I also run a test in Microsoft Word application on Windows environment, and the behavior is the same as Word app in MacOS. 
• However, if I select all the normal paragraph with a lot of lines (over 95 lines), this issue does not happen. 

I also find in all available documents from Microsoft, but there is no official confirmation for this issue. So, it may be by design of the Microsoft Word application if you press Enter multiple times like that scenario. 

In order to help you, I run a test for Microsoft Word in web version as well. Luckily, this behavior does not happen in web version. So, if you still want to do it for your works, I highly recommend that you may try it in web version of Microsoft Word as alternative method in this time. 

I hope that my testing can help you to understand this issue and I appreciate your sharing. Wish you a pleasant day! 

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This is a known limitation in Word. For any selection which is large enough, you will see options such as font and size grayed out, as if there were conflicting settings in the selection, even if there are no such conflicts.

I can't agree with others or reproduce. As shown below the font & size are clearly visible:

The one difference I notice is that your Font group on the Home tab as well as Tino's indicates that an East Asian Language is applied. That may be the cause.

As a side note I can think of absolutely no reason why the return key should be pressed repeatedly — especially not that many times.

This has been a problem for years. As a work-around, select one line or one paragraph; set the formatting you want; then, while that text is still selected, in > Home> Styles, right-click on Normal, and select Update Normal to Match Selection. Then use Crl+A to select everything in the document, and click the Normal style to make it all the same.