Not enough space to sleep properly:<a href="#_ftn1" name="_ftnref1">[1]</a>
If one’s Sukkah is very small to the point that it is difficult for him to sleep comfortably while inside, such as if he is unable to stretch his arms and legs and must sleep in a fetal position with his arms and legs bent inward, the Sukkah nonetheless remains valid.<a href="#_ftn2" name="_ftnref2">[2]</a>
<a href="#_ftnref1" name="_ftn1">[1]</a> Admur 640:7
<a href="#_ftnref2" name="_ftn2">[2]</a> The reason: As this is not considered a true pain, as it is common on occasion for people to sleep in a fetal position with the body and limbs bent. [Admur ibid; Rama 640:4; Terumos Hadeshen 92; M”A 640:9

Shulchan Aruch HaRav | 📄 The Validity of a Sukkah too small for sleeping

Not enough space to sleep properly:[1] If one’s Sukkah is very small to the point that it is difficult for him to sleep comfortably while inside, such as if he is unable to stretch his arms and legs and must sleep in a fetal position with his arms and legs