do you know if I can use a 8M/4Mb memory instead of the 2Mb, would it benefit the equipment in any way or am I forced to use the same size as the old memory.
No benefits, only hassle.
It can work if you only want to manually program the memory. You'll have to check how the extra address pin(s) are connected though. If you'd simply put the same data to the extra memory areas, only the unconnected pin might cause a problem.
If you're planning to use Keithley's flash programmer program however, the chances are it won't like it. For that matter, the same might be true for any random replacement. I remember seeing a list of different types (probably those are supported by the program), either in a txt or in the exe itself. It's better to choose one that is supported.
Edit: In fact there's a flshdata.txt file, the following are supported by the flash wizard. TiN's suggestion is included, so that should be fine.
' Number of Flash Parts Supported
47
' AMD29F040 (+5V, 4M)
&H1A4&,&H1A4&
' AMD29F002NB (+5V, 2M, Bottom Boot)
&H134&,&H134&
' AMD29F002NT (+5V, 2M, Top Boot)
&H1B0&,&H1B0&
' IS28F020 (+5V&+12V, 2M, Single Sector)
&HD5BD&,&HD5BD&
' AMD28F020A (+5V&+12V, 2M, Single Sector, Embedded Programming)
&H129&,&H129&
' AMD28F020 (+5V&+12V, 2M, Single Sector)
&H12A&,&H12A&
' M28F201 (+5V&+12V, 2M, Single Sector)
&H20F4&,&H2011&
' M28F201A (+5V&+12V, 2M, Single Sector, Embedded Programming)
&H20F5&,&H2012&
' SST28SF040 (+5V, 4M, 256byte page erase, byte write)
&HBF04&,&HBF04&
' SST29EE020 (+5V, 1M, 128byte pages)
&HBF10&,&HBF10&
' SST39SF040 (???????????????????? it's a new chip)
&HBFB7&,&H1F0B&
' SST39SF020A
&HBFB6&,&H1F0B&
'MX29F040C (+5, 4M)
&HC2A4&,&H1F0B&
' W29C020 (WinBond, +5V, 2M, 128byte pages)
&HDA45&,&HDA45&
' TMS28F020 (+5V&+12V, 2M, Single Sector)
&H89BD&,&H89BD&
' ATMEL29C020 (+5V, 2M, 256byte pages)
&H1FDA&,&H1FDA&
' ATMEL49F020 (+5V, 2M, Bottom Boot)
&H1F0B&,&H1F0B&
' ATMEL49F002T (+5V, 2M, Top Boot)
&H1F08&,&H1F08&
' ATMEL49F040 (+5V, 4M, 8 sectors)
&H1F13&,&H1F13&
' CAT28F020 (+5V&+12V, 2M, Single Sector)
&H31BD&,&H31BD&
' W49F002B (+5V, 2M, Bottom Boot) -> AMD29F002NB
&HDA25&,&H134&
' W49F002N (+5V, 2M, Top Boot) -> AMD29F002NT
&HDA0B&,&H1B0&
' W29C040 (+5V, 4M, 256byte pages) -> ATMEL29C020
&HDA46&,&H1FDA&
' BM29F040 (Bright Micro Elec/Winbond, +5V, 4M, 8 sectors) -> AMD29F040
&HAD40&,&H1A4&
' M29F040 (ST, +5V, 4M, 8 sectors) -> AMD29F040
&H20E2&,&H1A4&
' M29F0002NT (+5V, 2M, Top Boot) -> AMD29F002NT
&H20B0&,&H1B0&
' SST27SF020-70/90-3C-NH (+5V&+12V, 2M, Single Sector) -> TMS28F020
&HBFA6&,&H89BD&
' SST29EE020A (+5V, 1M, 128byte pages) -> SST29EE020
&HBF24&,&HBF10&
' V29C51002B-55J/90J (Mosel, +5V, 2M, Bottom Boot) -> AMD29F002NB
&H40A2&,&H134&
' V29C51002T (+5V, 2M, Top Boot) -> AMD29F002NT
&H4002&,&H1B0&
' V29C51004B (+5V, 2M, Bottom Boot) -> AMD29F040
&H40A3&,&H1A4&
' V29C51004T (+5V, 2M, Top Boot) -> AMD29F040
&H4003&,&H1A4&
' V29LC51002-90J (Mosel, +5V, 2M, 512byte sectors) -> AMD29F002NB
&H4082&,&H134&
' AS29F040-55LC/70LC/70LI/90LC/90LI (Alliance Semi, +5V, 4M) -> AMD29F040
&H52A4&,&H1A4&
' HY29F040AC-55/55I/70/70I/90/90I/12/12I (Hyundai, +5V, 4M) -> AMD29F040
&HADA4&,&H1A4&
' TE28F800C3B (Intel, +3.3V, 8Mb, x16, TSOP, 8-8kB + 15-64kB sectors)
&H008988C1&,&H008988C1&
' TE28F160C3B (Intel, +3.3V, 16Mb, x16, TSOP, 8-8kB + 31-64kB sectors)
&H008988C3&,&H008988C3&
' TE28F320C3B (Intel, +3.3V, 32Mb, x16, TSOP, 8-8kB + 63-64kB sectors)
&H008988C5&,&H008988C5&
' Numonyx 28W320FCB -> Intel 28f320C3B
&H002088BB&, &H008988C5&
' TE28F640C3B (Intel, +3.3V, 64Mb, x16, TSOP, 8-8kB + 127-64kB sectors)
&H008988CD&,&H008988CD&
' AMD29F400BB (+5V, 4M, Bottom Boot, x16, TSOP)
&H000122AB&,&H000122AB&
' AMD29F400BT (+5V, 4M, Top Boot, x16, TSOP)
&H00012223&,&H00012223&
' HY29F400BB (+5V, 4M, Bottom Boot, x16, TSOP)
&H00AD22AB&,&H00AD22AB&
' HY29F400BT (+5V, 4M, Top Boot, x16, TSOP)
&H00AD2223&,&H00AD2223&
' ST29F400BB (+5V, 4M, Bottom Boot, x16, TSOP)
&H002000D6&,&H002000D6&
' ST29F400BT (+5V, 4M, Top Boot, x16, TSOP)
&H002000D5&,&H002000D5&
' MX29F400CB -> AMD29F400BB
&H00C222AB&,&H000122AB&