SHOULD the employer be required? That's a subjective question that people could debate endlessly. The obvious reason to say "yes" would be that all employees should have uniforms that are reasonably professional-looking and comfortable. The obvious reason to say "no" would be the potential cost: if the existing uniforms fit 99% of employees, going out of their way to create a special uniform for one person could be a hassle and an extra expense. This would be especially true for a new employee. Do they have to spend a bunch of money to get a custom-made uniform for this one person, when the person might decide they don't like the job for any number of reasons and quit within a short time?
What you might do about it is more answerable.
If the uniform you were given is clearly way too large, if you could turn around in the pants three times and they wouldn't move, etc, you could certainly approach your boss and politely say, Hey, this uniform really looks unprofessional. If there's a legitimate safety issue, if you really are routinely called to work around machinery that baggy clothes might get caught in or some such, bring that up too. Let me emphasize "politely": Going in screaming about the injustice and discrimination or whatever can make you look very foolish if it's an issue no one else has even noticed and they are all happy to remedy. And if you're new on the job, you don't want to make everyone's first impression of you that you're the angry screaming person who is impossible to get along with. No way is that good for long-term job prospects. And if safety isn't really an issue, don't invent unlikely hypothetical safety problems so you can make the problem sound more serious. Your boss will likely know that you're inventing problems that don't really exist, and that won't work to your favor.
Anyway, you go to your boss and describe the problem. If he says, "Oh, I hadn't noticed, of course we'll deal with that", then great, problem solved. If he gives some semi-satisfactory answer, like, "We'll see about getting you a better-fitting uniform when you've been here for six months", I'd accept it.
If he tells you to shut up and go away, then you have to consider your options. Maybe you could win a sex discrimination lawsuit. I don't know; you'd have to check with a lawyer. If it's a big enough deal to you, you always have the option of quitting. You can decide to put up with it.
Perhaps you can suggest practical alternatives. Like, what if you pay a seamstress to alter your uniform out of your own pocket? (Or do it yourself if you have the skill.) Would that be acceptable to the boss? I don't know how distinctive the uniform is. If it's basically just green pants and shirt, can you just get green pants and shirt at a department store that fit you and wear those? Etc.
Also, bear in mind that the situation you describe presents the employer with a catch-22. If they create a women's uniform that is a noticeably different style and cut than the men's uniform, some number of women will complain that they are not being treated the same as the men and this is sexism. If they expect the women to wear the same uniform as the men, some number of women will complain that this doesn't fit well and this is sexism. Unless you like confrontation, try to give your boss a solution that doesn't put him in a bind.