Avoid open oversize position EA for MT4 platform

i want to create an EA to avoid accidentally opening oversize positions. It means, for example, i just plan to open 0.1 size of position on XAUUSD trading, but by unknown reason (may be pressing a wrong button) I open 1 size of position, which let me to get much higher risk.

I am using Vantage, i trade NAS100.r, CL-OIL, GBPJPY, XAUUSD (i am using Vantage as my broker), i set the position of each item as follow:

extern double NAS_posit = 1

extern double CL-OIL_posit = 0.1

extern double GBP_posit = 0.2

extern double XAU_posit = 0.1

Obviously, this EA will help me to trade not over the above size of position.

For coding, it is a must to use extern , cos i often change the position, it is more convenient for me

I will put this EA to the Window version of MT4 in a VPS platform and keep it running, however, basically, i only make my trades in mobile MT4, so for the EA, i hope it can reject all over size trades, if not, it should:
1.  cancel all over size trades (I always use pending order, so it is ok if the EA in checking Ontick) 

2. If I accidentally open a market execution trade, the EA needs to help me to close it immediately.

Finally , since i am a rookie of programming, i hope you can give me simple description on important line of code.