Help needed to knit a larger size of baby pattern

You would need to completely redraft the pattern. The largest size is 20" chest. If, say, you now need a 24", then you need to add an inch each side of the front and each side of the back. Do a test square to see how many stitches you knit to the inch, then add this number each side, front and back. Bear in mind you will also need to make it longer, but that should be relatively easy to calculate by just measuring the child.
You will also have to work a few extra stitches into the neck shaping. Probably best if you draw the whole thing on a piece of graph paper (but remembering, of course, that knitted stitches aren't exactly square, so the finished draft on graph paper won't necessarily like in proportion).
And then, of course, you will have to adjust the sleeves accordingly.
Of course, you could always take a chance and knit the whole thing using, say, Aran yarn or even chunky knit with the appropriate size needles. Just follow the pattern exactly, adjusting the length where necessary.

...won't necessarily look in proportion

You will have to work out stitches per inch and increase accordingly.

As it's in different sizes you could use the number of stitches per size as a guide and use a chart or write out your own pattern.

I once knitted a cardigan for DGD by using bits from 3 different patterns, but I had to work it out and write it all down first.
It worked out fine but I don't think I'd do it again.

This one's not quite the same but goes up to age 2 and looks quite unisex!
www.ravelry.com/patterns/library/ribbed-baby-jacket

Thanks for the suggestions. I may try the thicker wool and bigger needles for starters.

I like that jalima I shall check it out.

You could perhaps not do the collar if you don't like it.

I have just knitted waistcoats with a similar neckline, you could choose either a shawl collar or just a wide ribbed edging.

Ooh the waistcoat sounds lovely. Is it a Ravelry pattern?

I'm just finishing this lovely boy:

www.knitrowan.com/designs-and-patterns/patterns/teddy-bear

It's a Sirdar pattern, friend has borrowed it now and I can't remember the number and can't find it on their website.

It was a girls' pattern though.
www.johnlewis.com/sirdar-supersoft-aran-leaflet-2409/p955117?sku=232868846&s_kwcid=2dx92700013398637334&tmad=c&tmcampid=2&gclid=CPX07reTsNACFdS4GwodqnoK9w&gclsrc=aw.ds

I must remember to call it a 'gilet'

Teddy is very cute!!

Oh I'm so jealous! I used to knit such a lot, especially in the winter months, but in the last 4 or 5 months I've developed osteoarthritis in my hands, coupled with carpal tunnel syndrome which means my hands go numb when held in the knitting position. So I've had to give up knitting and crochet altogether

That's sad Indinana
After DM had strokes she used to say 'I'd be happy if I could still knit'

I know how she felt. I never thought I'd miss it so much, but I sit watching TV on the long winter evenings and my hands are idle in my lap. It's hard getting used to it.
I'm thinking of getting a knitting machine, then at least I'll be able to make knitted things. Still won't be able to do it while watching TV though!

Teddy is cute Jalima. I've never knitted a toy before and the hardest part was putting a face on him. Every time I did, he ended up looking cross, or sad or just plain odd. I finally got him to smile!

Meant to say thanks for the gilet pattern link.