What size?

What size of bag do I need?
Measure your item, length + height = your minimum length of the bag.
Measure width + height = your minimum width of the bag.

And then add at least 10% to be safe.

Example:

A shoebox with length 39 cm and height 17 cm: 39 + 17 = 56 cm.
Your bag must then be at least 56 cm long for it to fit, add at least 10%.
56cm + 10% (5.6cm) = 61.6 cm long bag + flap.

You calculate the width in the same way, say the box is 21 cm wide and still 17 cm high: 21 + 17 = 38 cm.
Your bag must be at least 38 cm wide for it to fit, add at least 10%
38+10% (3.8) = 41.8 cm wide bag.

If I were you, I would look for a bag that is at least 42×62 cm.

How do I measure a child's overall?

Simply fold the child's overall as best you can and try to get a circumference.
Circumference "across the width" divide by 2 = minimum width. Circumference "lengthwise" divide by 2 = minimum length.

Example:

The overall is 86 cm in circumference when folded, then you need a bag that is 43 cm wide for it to fit. Add at least 10% (4.3) which gives 47.3 cm wide.
The length in circumference becomes 128 cm, then you need a bag that is 64 cm long + flap for it to fit. Add at least 10% (6.4) which gives 70.4 cm long bag + flap.

Sometimes we get questions about the volumetric weight on packages (bulky items), and there you want a bag that is under 25 cm in width, but then we usually respond that:
If you have a bag, for example, 27 cm wide, then it's considered bulky according to the postal service.
BUT if you put something in the bag that is thicker than 10 mm, then it's not considered bulky!
The width of the bag became smaller because of what you put in it.