First of all, don't use the spare for your measurements. The problem with using the spare is, while it obviously fits your car, it usually won't be made to the same spec as what is on your vehicle for normal use. This is due to several considerations, mainly getting it to fit into the hole where it's stored.
Your best bet is to go to a website which will tell you exactly the fitment of the wheels which will go on your car. For instance this website shows all of the dimensions for the different years of BX's (I used 1990 because it was middle of the road and had to go with something). This website shows the BX has several different engine sizes (1.1, 1.4, 1.5, etc.). Along with it, there are several different wheel/tire combinations which are available.
There are three measurements which are the same:
Bolt Pattern (PCD) 4x108
Thread Size (THD) M12x1.25
Center Bore (CB) 65.1
Each of these measurements are metric, so in MM. (The code in parenthesis is what is on the website I gave you, to help you be able to understand their charts.)
The rim diameter, width, and offset are a little bit more tricky. There are several different ones listed on here. The easiest way to figure out which one you have is to look at the markings on the tires which are on the car. However, you could use any one of the following, as the BX was equipped with each:
185/60R14 5.5jx14 ET18
185/60R14 6jx14 ET15
185/55R15 6jx15 ET15
195/50R15 6jx15 ET15
205/50R15 6.5jx15 ET15
195/45R16 6.5jx16 ET15
205/45R16 7jx16 ET15
As you can see, there are seven different rim sizes which you could use (assuming your car is a 1990, but I'll also venture to assume the sizes didn't change much between the years the BX was available). You need to remember with this, though, the tire width dictates the size of the rim. Wider tire; wider rim. As for the measurements which are given, you can find the tire size explanation pretty much anywhere and see what they mean (tread width in mm; aspect ratio; rim size in inches).
The wheel sizes need a little bit more explanation, though, due to the fact measurements given for European sale is different than what's given here in the States. The sizes given are thus (using the first wheel size listed as the example):
5.5 Wheel width in inches
14 Wheel diameter in inches
ET18 Offset in MM
In your case, the offset listed is a positive offset. There can also be a negative offset for some wheels. Measuring the offset can be quite tricky and is the hardest thing to get right. Using a website which has all of the figures available for you in the first place is a much easier way to get any of these measurements, because they will for the most part be accurate.