Size can be varied by repeating less or more pattern repeats across and down.

Heaver weight blankets can be made using:
Aran (5mm needles)
Chunky (6mm needles)
2 x strands of double knit yarn (6-7mm needles) - experiment with mixing colours, textures and styles of yarn for some interesting results..

To keep to pram size you should reduce the number of pattern repeats across and down.

I would recommend checking your tension and making the appropriate calculations/adjustments to acquire the desired size.