First of all, congrats on your engagement!!
The details of your ceremony often depend on whether or not you will be incorporating a particular religion or cultural tradition. For example, Catholic wedding ceremonies tend to be very long as they have a lot of traditions and formalities(e.g., 1-2 hours); while many other ceremonies, are traditionally shorter (e.g., 20-30 minutes).
Thus, the first question to figure out is whether you are having a ceremony that incorporates a certain religion or culture, if you are, then you should ask your officiant what the mandatory parts of the ceremony are, and how long they normally take.
The next question, is what are the particular details that you want. If your religion/culture does not mandate certain events (e.g., mass); you can likely choose how long you want your ceremony to be and discuss it with the officiant. If your religion/culture does mandate certain details, then you should think about these extra choices as adding time to the minimum length of the ceremony). Think about:
-how many bridesmaids/groomsmen will be walking down the aisle? how many flower girls? ring bearers? family members?
-will they be walking together or separately? what kind of spacing do you want between them? will you wait for one to reach the alter before sending down the next?
-do you want the officiant to talk a lot about you and your fiancee, or do you want something short and sweet?
-do you want to have a lot of readings (biblical, poems, etc.)?
-do you want someone to sing a song during your ceremony?
-are you adding any special cultural elements that may stretch the time?
-are you planning on waiting for all of your guests to arrive? or are you planning on starting at the exact invitation time?
-does your ceremony site have restrictions on starting the ceremony on time (usually unless you have a wedding planner, or there is a penalty or a time that you have to get kicked out as a major motivation, ceremonies will start late)
-how far is the wedding hall from the ceremony site? are your guests going to be driving around rush hour?
As you can see, a lot of thought can go into this process, and it is not a decision that you should have to make right now. You might want to tell the reception hall that you estimate that the wedding will start around ___pm, but that you still have to talk to your officiant and make decisions about your ceremony, and ask if it would it be a problem if closer to the wedding you change the start time to a couple hours earlier or later...and if you can change it, when would be the latest you could tell them about the change. This would be your best option, because you may not make all the choices for your ceremony until sometime closer to the wedding, and this way you don't have to be forced to rush these decisions now.
By the way, if your wedding hall says that you can't change the time within a range of a couple hours, I would wonder whether they plan to book another wedding back to back with yours, and whether that is something you are ok with.
If your reception hall won't let you change the time closer to the date, but they are flexible with when you arrive (e.g., they are not charging you by the hour), you may be better off telling them a time on the earlier side, just in case.
Also, when you tell them the start time for the reception, make sure to also check what the start time is for decorating, so that your vendors have adequate time to prepare the room. (This by the way is why they should be able to be flexible with your time, the room should be decorated ahead of time so it is ready when you get there--so they can't do anything with the room anyway whether or not you are there--the only real variation is the staff--and I would be shocked if the staff is given the schedule almost a year before the wedding).