ForceChoke wrote:That's a good question. The decal needs to be about the size of the center squar/ rectangle on a drop pod door.
For those of us that don't own a drop pod, you need to give us an actual size.

Grab a ruler and measure it and measure how big the decal needs to be to look the way you want it.
40k scale is weird, it doesn't really match any real life scale all that well and even if it did I have no bloody idea how big that door is
I plan to try stencils fist they have worked with limited success in the past. I hear good things about contact paper. However if that fails. Decals.
Do you have an airbrush? I cut stencils from painters tape quite successfully, specifically Tamiya tape... don't just use cheap "masking tape" that you buy from a supermarket as it'll probably be too sticky and tear up the paint, look for painters tape or masking tape that is actually designed for going on to paint. I also use vinyl sheets for masks, but they are a bit harder to work with and it needs to be very weakly adhesive vinyl otherwise it'll tear your paint up.
I suppose the plane would need to be 40k mini scale. I want to convert it to a downed ork. Tri plane. Head fisrt in the sand. Some grots. Orky paint. Bullet belts. Etc..
As I said you can definitely buy the decals by themselves if you don't want to buy the whole plane.
If you do want to buy the whole plane, 1/48th scale is *usually* a good starting point, but it depends on the plane.
40k models tend to have really fat features, so if you want a 48th scale plane to look the part you usually have to convert it so the fuselage is fatter.
But you'll need to check if 48th scale decals will actually match your drop pod door, you may want to go down to 72nd scale if the doors are small or up to 32nd scale if the doors are big (72nd scale might be about the right size for a grot plane, 32nd will probably be too big).