3D ultrasounds

Adina • I`m 24 with a 2 year old boy and another on the way. Married for 3 yrs.
I got a 3D ultrasound at 21 weeks but baby was so small & skinny (still cute of course) but he wanted to hide up near the placenta & have his back facing towards the device. today I am 25 wks.
I was wondering what the cut off is? When are they too big for the sonographer to see them? I would like to see my baby once more when I'm in the third trimester so what's the latest I can get another?