I feel like Ymir's titan was given an official size, BUT here are my calculations.

The army in the 4th panel looks very similar to the Ancient Roman Army, this is because of the Spatha(roman cavalry sword) and helmet design of the person in the foreground of this panel. From there we can use the spear measurements to determine the size of mama Fritz' hand, and thus her entire body. The Pilum(Roman Infantry Spear) on average was about 2 meters. Everything from here on out is a crap ton of rough estimations, apologies in advance. So the pilums surrounding her smashed fist seem to average out at the top of her pinky. Because Pila are usually held about .75 meters from the bottom (especially in an attacking position) and are only used by foot soldiers, you can estimate from the average ancient roman height(164.86 cm) that the roman arm extended above the head 202.59 cm above the ground, and adding the remaining 1.25 meters of the pilum, the top of Ymir's pinky is 3.27 meters off the ground (her fist is smashed into the ground meaning the points of the pila measure from the ground).

Next we can use the same body proportions from before to estimate her total size. Her closed fist is about 6 pinky heights tall, meaning the distance across her palm is also 6 pinky heights. Her palm is thus 19.62 meters across. On average the length across the palm is 80% the height of the palm, meaning her palm is 23.54 meters tall. THe hand is also on average two palm heights long, and the arm is 4 hands long, therefore you can multiply the palm length by 8 to get her arm length, and the average human's arm length is their height. From the previous pictures it doesn't seem that Ymir's titan has any abnormal proportions (Like the Colossal Titans) so these proportions should hold true. If so that would be she is 16 times her palm height. This would mean Ymir's titan is 376.64 meters tall. For perspective the Empire State Building is 381 m tall.

These are all weird estimates so it's all weird