According to Shinobi no Ittoki Fandom, Ittoki Sakuraba is the main character and he is a “boy” that is a first year in highschool. The plot of the anime revolves around his life and how he finds himself in a lot of trouble after discovering that he belongs to a lineage with ninja origins.
Since the plot mentions Ittoki is a “boy” and a first year highschooler, this gives us an idea that his age should be below 18. Since he is a “first year” in highschool, we assume that he should be somewhere around the age of 17.
There hasn’t been yet any mention about Ittoki Sakuraba’s height. We estimated his height by visualizing him in this anime clip on Youtube. In the timestamp 0:01, we can see him enter the house using the apartment door. According to this Quora discussion, doors in Japan are relatively smaller compared to Western doors. Typically, Japanese doors should only measure around 6 foot tall. As can be observed on the anime clip, there seems to be only very little gap between Ittoki’s head and the doorway’s upper frame. This leads us to a conclusion that Ittoki Sakuraba’s may be somewhere around 5 feet and 11 inches tall.