As a trainer, this is a concern for me for several reasons. At this age, your foal is in a very impressionable stage. She will either learn to accept being handled with a confident and knowledgeable handler, or she will learn to evade, kick, turn her butt, etc if you continue to ignore her and not work with her through this phase.
My other concern is for you to be working with her by yourself. I do NOT recommend this. Not only because you stated she is getting too big, but because the assistance of another person when teaching a foal to halter and handle makes all the difference in the world. From your post, I did not get the feeling that you would be able to successfully accomplish your goal alone.
First what I recommend that you do is contact someone who is experienced in working with foals. Then, the two of you together should start by working with her in a small area such as a safe stall. Remove any objects in the stall for those sessions, such as water buckets, feed tubs, toys, etc. and anything that she may crash into when you work with her.
You and your other handler will have to go in confidently and slowly approach her together, working towards putting her in a corner to finally capture her. You must first decide between the two of you who will be holding her, and who will be doing the hands on work. One of you will be holding her from in front of her chest to behind her rear, and the other person will need to be from about the shoulder area to her head area in order to contral her front end.
I will honest with you, be prepared to be knocked in the teeth or nose, possibly kicked a couple times, reared up on, etc. You many even clash a few times into your handler (and have a good laugh about it later on). If there is any way for a third person to come into the picture, you can try that as well.
Keep in mind that you are NOT to chase the foal around the stall, there is no point to that. If you are chasing the foal, then the both of you are out of position and not working to "corner" her properly. If she is running around, simply park yourselves still for awhile and let her run herself down.
I specialize in behavioral reformation, and i'm often called upon to come out to a barn with a foal who no one else can work with. I go in very prepared with another handler- and a plan. Yes I may get kicked, stomped on, bruised, etc, but I get amazing results from approaching the situation head on and not backing down. It is the intial fear and insecurities of the foal to try to flee, that is natural. Some foals have learned to many tricks that they are quite the challenge and can be dangerous, nontheless, they need the lessen just as much.
You may have good days and bad days with this foal. She may do well one day,then surprise you the next. I would make a schedule with your second handler to come and assist you on a daily basis for about two weeks to get this baby handled, halter broke, and getting her accustomed to things like basic grooming, etc. Then, work with her alone and have your handler show up every so often to reassist you and make sure your foal has been respecting her lessons....I hope this helps, let me know how things go...:)