I am asked to do a large print of an A1 size poster which is 840mm x594mm,so would it be wiser
to half those dimensions given,or even 1/4 of that size in order to avoid my maching going slow?
would the printer still understand how hes meant to go about,if i specify the actual dimensions?
It's impossible to answer your question, because you haven't said how much computer power / resources you have.
A powerful modern machine could easily facilitate work on an A1 image of, say, 7000 x 10000 pixels at 16 bits/channel and a number of layers without "going slow" as you put it. That's only 70 megapixels.
On the other hand, a small / older system could well be impacted by working on a relatively large document.
But your choice of resolution should really be based on the content of the work, not on your computer. If it's a highly detailed photographic print that will be examined up close - even though it is large - you might want to use a higher resolution than a poster that will be viewed from one or several meters distance. If you really don't know how it's going to be viewed, opt for the higher quality.
Why not just try it? You could always downsample the document later if things just get unworkable.
If you should decide to work smaller, A3 at 300ppi would come out as 150ppi at A1 which for a poster to be viewed at a distance larger than »arm’s length« should be fine.