Hi and welcome! Your question really needs a bit more context -- are Evie's parents fully health tested, and is the breeder responsible and reputable, with established health tested breeding lines (and not just 'health-tested' by the vet, which doesn't meet the basic criteria for proper health testing in this breed ... cardiologist heart evaluation, a number of DNA tests, MRI, specialist eye tests etc)? If you're working with a reputable breeder, s/he should be able to tell you why this puppy is so undersized (eg , is she a litter runt? If so, she will likely catch up in weight to others in her litter). Unfortunately though, a lot of disreputable breeders aim to produce extra small dogs which are way under the breed standard and they do this by breeding already undersized dogs which are often runts or maybe, small due to undisclosed health issues.
Why is she going to be blind in one eye? Is this due to an underlying health condition like dry eye/curly coat syndrome? An accident?
If this is a rescue dog, or you don't know much about the breeder (a red flag) then you are of course dealing with many unknowns. It would be hard to say why she is so small but the cause could be health issues such as liver shunt (
https://www.petmd.com/dog/general-health/liver-shunts-dogs-what-you-need-know). Runts do sometimes have health issues too, as opposed to just being the small one in the litter at the start, which often changes as they grow).
A reputable breeder will discuss the realities of SM with potential buyers and have testing certs and at least two-generation health results (parents are good, grandparents far more meaningful). Syringomyelia is very common in the breed though not always symptomatic. It can be managed but can be a lifelong cost. Many of my cavaliers over the years have had some degree of CM/SM. All have been managed with meds, but of course this isn't acceptable for the breed and breed clubs and breeders need to step up and start to do. more. The Swedish club has begun a carefully planned and supervised outcross programme to try and rescue the breed from both the numerous health issues, and the breed's close inbreeding.
If you haven't yet read the information pages at Cavaliermatters.org on buying a puppy and the breed health issues, I'd recommend doing so as the site lays these out, is easy to understand and informative. Some tests refer to the UK bodies that issue results but there are similar in the North America and elsewhere. Cavalierhealth.org has lots of international detail.
If you are dealing with a breeder of unknown reputation who is selling a very undersized puppy that will be blind in one eye and hasn't discussed why the pup is like this, or breed health issues like SM with you, or offered you extensive health and lineage paperwork on Evie's parents (at a minimum), I'd walk away from the breeder, as hard as that may be.