Needles... help!

If I knit on needles 1.5 mm smaller than recommended in pattern, for a ‘jacket’ which I’d prefer to wear as a cardigan, i.e. Slightly snugger fit, will I end up with something far too small.? . The overall size is said to be length 29" /width 74" using 5.5 mm needles for the body, and 6.5 mm needles for the rib. If I rib in 4mm and do the body in 5mm will there be an appreciable difference in size?

Welcome to KnittingHelp!
When you knit a gauge swatch with the smaller needles, you’ll be able to determine the stitches per inch that you’re getting and calculate how many inches smaller the cardigan will be. Then you’ll know whether you need to add stitches or perhaps knit a larger pattern size.
The needles given in a pattern are a recommendation for the “average” knitter, whoever that is. The needles you will need to use to get the given gauge or a tighter gauge depend on how loosely or tightly you knit.
Which pattern are you looking at?

If I were to switch to slightly larger needles, say 1mm larger than I’m using, ( now half way up the back doing stocking stitch), would this be ok? I am a little anxious the cardigan may be slightly too short on the present scale.

If you switch needle size it’ll show especially in a pattern like stockinette with a light color yarn. A dark yarn or a multi-color might not show as much. The piece is also going to become wider. If you’re worried about length, can’t you add rows, say before the armholes?
You can try switching needles and see how noticeable it is…

Yes, thank you for this. My pattern (Kaffe Fassett) is a series of tall diamonds, so lengthening the cardigan will be a matter of working out how to extend the grid, but it should possible. I will give it a try. Most grateful for your prompt reply.

the ribbed section all round the bottom of the jacket is too wide and
loose. is there any elastic thread i could weave through the rows to
gather it in and make it grip?

I’ve used thin elastic thread which is available in sewing supply stores. You can double it if necessary. Thread it through on the wrong side every couple of sts, maybe along 3 or 4 separate rows. It works especially well for a pullover but you can try it for a cardigan too.

thanks for this. is it possible to use a sewing machine to stitch the

ive looked on line. is it described as ‘beading thread’?

I don’t think beading thread is elastic. My sewing store sells elongated spools of elastic thread. It commonly comes in black and white but you can find it in limited colors too. Yarn stores may also sell some.