Depend the current area have enough sitting for the aluminum, if not you may need to hack the grill out, or maybe add some brick/concrete to let the windows sit on.

Usually calculation is round up. 3.5 or 3.8 use 4', 4'8" use 5'

Few things that you need to consider.
1. Is the laundry area located on the ground floor (landed property) or in a condo (above ground floor)?
Ground floor landed property.
If yours is on the ground floor, they can easily install from the outside. Else they will have to take out the grill to be able to install the window if your place is not on ground floor.
2. Is the plastering around the grille opening perfectly square? Cos the window will be square and if the present opening is not, then there will be gaps at some of the edges.

If you mean the edges whether comes up to a square, I believe yes. But if mean whether the cement area if perfectly 90 degrees to the grilles, I think no. It's slightly slanted. Better if I take a pic tomorrow.

3. Normally the windows frame will be installed with some portion embedded in the plastering. Avoid water seepage and also there will not be any gaps between the frame and plaster.

As for your concern about the alarm wiring. Mostly the alarm wiring are running on the inside and connected to the grille. You will be installing the window on the outside. So, the chances of damaging the wiring is very slim.

Which means as long as they don't remove grille or hack the inside, should be okay right?

There is this shop 'kaca and aluminium' who gave me a quote at RM20 psft. His shop is in damansara uptown. Sorry i can't recall the name of the shop. It's in the same row as a lot of lighting one and maybank. Beside the shop is a thai restaurant.

Maybe u can check it out. I did ask him whether can come do measurement or not. He said 'price OK or not 1st? OK then only we come talk about measurement'. ha ha ha...

Lowlowc..ya nowadays ppl do business all super rude one. Ask abit also cant.. haven't survey other shop yet but it seems his price so far is reasonable. I used his service before for frame work. if his price cheapest will let him do otherwise won't ...haha coz sure bad service

I also entire house windows from few windows supplier, they too tell me per sqft price ask me calculate myself. If accept only go measure and discuss detail. That is very standard practice. I don't think is rude.

Avoid Sliding Windows for outside facing windows.
The sliding groove collect debris. - (repeating myself here) 

I only asked if price include installation and is it they will come n measure the exact size, the person replied 'u confirm the price OK 1st la only ask this, if price not OK ask is useless'

wanna ask more clearly cannot meh?

lowlowc, i haven't ask him to do windows yet so i don't know.

---

Added on August 18, 2011, 10:01 pm

aren't all windows facing outside? what do u mean by outside facing windows?.. i dont understand...

Provide opening = enough space for the windows. Or they may charge you removal of the grills or old windows.

Plaster = After brick work, you need to put cement to make it like normal wall, this is call plastering.

Usually window frame are install after brick work, because they need to mount the windows frame holder / bracket to the brick, then the wet work contractor will do plaster. This look nicer.  If you have already plaster they may need to hack and put the bracket and plaster back. So this may not turn out to be nice  and may charge you extra.