Should men get their hair equally cut in sides and middle or can they leave it short in sides and long in middle?
I have seen some men get their hair cut equally on whole scalp.
Leaving it noticeably longer in some areas compared to others is what is known as qaz' (Arabic: قزع), which the Prophet ﷺ forbade us to do. Either shave it all or leave it all:
حَدَّثَنَا أَحْمَدُ بْنُ حَنْبَلٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، حَدَّثَنَا مَعْمَرٌ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم رَأَى صَبِيًّا قَدْ حُلِقَ بَعْضُ شَعْرِهِ وَتُرِكَ بَعْضُهُ فَنَهَاهُمْ عَنْ ذَلِكَ وَقَالَ: احْلِقُوهُ كُلَّهُ أَوِ اتْرُكُوهُ كُلَّهُ
The Prophet (ﷺ) saw a boy with part of his head shaved and part left unshaven. He forbade them to do that, saying: Shave it all or leave it all.
In Fat'h Al-Bari (Arabic: فتح الباري شرح صحيح البخاري) by Ibn Hajar al-Asqalani (Arabic: أحمد بن علي بن حجر العسقلاني) in a chapter dedicated specifically to the topic of qaz', he explains that qaz' means either shortening the hair above the forehead and leaving the back long, or shortening the back and leaving the hair above the forehead long. It also includes shortening the sides and leaving the middle long, or shortening the middle and leaving the sides long. In all cases, qaz' refers to noticeably longer hair in some areas when compared to other areas. The hair does not have to be cut exactly equal on all sides, but be relatively similar in length.