I90 Controller providing OJT described a loss of separation event listing physical limitations when observing a developmental as a possible contributing factor.

Narrative

While training on ELA Arrival (X) a CL60 landing HOU was on a vector to transition from the COACH Arrival to land at HOU. The aircraft was descending from 11;000 FT to 6;000 FT. We also took a hand off from West Departure on a LR55; an aircraft that was transitioning from HOU to land on Runway 8R at IAH. Usually traffic between HOU and IAH landing Runway 8R doesn't get handed off to ELA. They and are left at 5;000 FT and go straight to Final Center. We had a brief discussion about the options we could use in transitioning this aircraft to IAH. The developmental decided to climb the aircraft to 7;000 FT and assigned a vector to pass the aircraft behind the CL60. From my incorrect perspective; which is a direct result of having to sit well behind the developmental due to the limited amount of space between the control positions at I90; I advised that I thought he/she could turn the LR55 direct DPLOY. After the developmental turned the LR55 direct DPLOY; I realized that it was more of a converging heading than a pass behind heading. I waited a short time to see if he would recognize the problem and when he didn't I told him to tell the CL60 to give us a good rate of descent to 6;000 FT. When this appeared not to be working I then told him to turn the LR55 to the left. He ended up giving the aircraft 2 turns. I believe the first turn was to 340 degree heading and then finally to a 260 or 270 degree heading. When I realized that the LR55 could not make this turn in a timely manner due to his indicated airspeed of 300 KTS I then instructed the developmental to see if the LR55 had the CL60 in sight. The LR55 reported the CL60 in sight prior to the loss of IFR separation but not before we could complete the required 7110.65 phraseology. When the CL60 was informed of the traffic maintaining visual separation with his aircraft he informed us that he had responded to an RA. We then informed the Supervisor of the RA and the close proximity of the separation. Recommendation; finish the new I90 TRACON and allow more space between the RADAR scopes so that OJT instructors can sit next to their trainees and not be forced to sit behind them.